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BedokFunland JC's A Level H2 Chemistry Qns (Part 2)

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    • Originally posted by hoay:

      What is the composition of a buffer solution of pH 7.4 made by mixing 0.1 M NaH2PO4 and 0.1 M Na2HPO4. [Ka for NaH2PO4 = 6.22x10-8 M).

      How to approach this question because evrything is given which can be pleaced in the Handerson-Hassalbach equation. But what we will get....?? Help please...

       


      From the Henderson–Hasselbalch equation, plug in the pH and the pKa value, to solve for the mole ratio of conjugate base (HPO4 2-) to acid (H2PO4 -).

      Since the molarities for both are the same, the ratio of moles = ratio of volumes, and that's the answer ie. 'composition' of buffer solution.

      Edited by UltimaOnline 19 Nov `16, 5:50PM
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    • Originally posted by gohby:

      Hi UltimaOnline,


      Here are some DHS MCQs on which I would like seek your help:


      DHS14/P1/12 [Ans: D]


      Remarks: I do not understand the titration curve. My understanding is as follows - the first unlabelled equivalence point between A and B signifies the end of neutralisation between maleic acid and sodium hydroxide, and at the point, only -O2CHCHCO2- is present. However, why is the second equivalence point is at C, (when an additional 10cm³ of maleic acid is added from the first equivalence point), instead of an addition 30cm³ of maleic acid (given that there is 30cm³ of -O2CHCHCO2- at first equivalence pt)? Can I confirm that at D, the only compound present is HO2CHCHCO2-? So where is “mixture” to resist the pH change? If there were an option further down the graph from D, I would have picked that, but D doesn’t seem correct to me.


      DHS14/P1/20


      Remarks: I understand that elimination is out because it wouldn’t yield a product which rotates plane-polarised light. I also understand that both SN1 (since an optically pure enantiomer of 2-bromobutane was used since an optically pure enantiomer of 2-bromobutane was used) and SN2 mechanisms will yield an optically active product. Am I supposed to draw any inference from the reduction of the angle of rotation or is that just a red herring?


      DHS14/P1/25


      Remarks: I can understand that C is not the answer because it cannot be deduced from the information above. However, how do I know whether C-16O bond is stronger than C-18O bond?


      DHS14/P1/28 [Ans: C]


      Remarks: I’m rather confused here. Do the arrows in the choices signify a successive addition of the compounds without any discarding of the aqueous layer? Why do I add water in the first and last steps and how do I go about ascertaining the sequence of the addition?


      Once again, thank you very much for your help! :)



      Hi Gohby,

      DHS14/P1/12. Since you're adding a weak acid to a strong acid, hence maximum buffer capacity is double equivalence point instead of half equivalence point. Being a diprotic acid, there are of course, 2 maximum buffer capacities. The 1st being equal molarities of HA- and A2- (which occurs at [10+20]/2=15 cm3), and the 2nd being equal molarities of H2A and HA- (which occurs at 40 cm3). Clearly, the more effective buffer occurs at equal molarities of H2A and HA- (ie. weak acid and amphiprotic species) rather than equal molarities of HA- and A2- (ie. amphiprotic and strong base), as illustrated by the larger horizontal region of the titration curve. Besides, 15cm3 isn't even an option in this MCQ.

      DHS14/P1/20. If it were SN1 only, and the reaction was complete (as implied), then the product would be optically inactive. If it were SN2 only, and the reaction was complete (as implied), then the product would rotate plane polarized light by 13.5 degrees in either direction. Hence a mixture of both SN1 and SN2 must have occurred.

      DHS14/P1/25. Option D is correct because the reactant is cyclic, hence there is only 1 hydrolysis product, not 2. Addressing your specific query : heavier isotopes form stronger bonds. This is Uni level Chemistry, and for A level purposes, Cambridge can still tempt the students with beyond-syllabus MCQ choices, to test if the student is sufficiently proficient to recognize the within-syllabus correct MCQ answer, and disregard the other options.

      DHS14/P1/28. Problem is alcohol, being part non-polar and part protic polar, is miscible with both water and alkyl halide. Addding [water and excess conc HCl] serves to protonate the alcohol, which then migrates into the aqueous layer. Discard the aqueous layer by decantation. Add excess Na2CO3(aq) to remove excess HCl(aq) in the form of CO2(g) and NaCl(aq). Add S2O3 2-(aq) to reduce all the I2(s/aq/alc) to I-(aq), forming S4O6 2-(aq). Add water to remove all ionic impurities, then carry out decantation to leave behind the alkyl halide.

      No prob, Ghoby! :)

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    • Originally posted by supercat:

      Hi, could I ask a question about ionic equilibria?

       

      In JJC 2014 prelim mcq Q10,

      Maximum separation occurs when IP = Ksp of Fe(OH)2 right? With the [Fe] being 0.05, I can calculate [OH-] but I keep getting pH of about 7. May I ask where did I go wrong??

       


      A researcher accidentally mixed 50.0 cm3 of 0.100 mol dm–3 MgSO4 solution with 50.0 cm3 of 0.100 mol dm–3 FeSO4 solution in the laboratory. It is suggested that the metal ions can be separated through selective precipitation by adding solid sodium hydroxide. At 298 K, the solubility product of Mg(OH)2 is 1.8x10^–11 mol3 dm–9, and that of Fe(OH)2 is 4.1x10–15 mol3 dm–9. What would be the pH of the resultant solution when maximum separation has occurred?

      Your error is assuming maximum separation occurs when Qsp = Ksp of Fe(OH)2, when it should be Mg(OH)2, ie. the more soluble ionic compound. You have to add as much OH- as possible to precipitate all the less soluble Fe(OH)2, without precipitating out any of the Mg(OH)2. Hence at that point, Qsp = Ksp of Mg(OH)2, not Fe(OH)2.

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    • Originally posted by baby6vat:

      what is the product when butene reacts with IBr

      ?

       


      Major product is 2-bromo-1-iodobutane, minor product is 1-bromo-2-iodobutane.

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    • Originally posted by supercat:

      Hi, I have a question from MI 2014 prelim P1 Q24

      Why are there only 2 optical isomers for R? Is it due to internal plane of symmetry? I counted a total of 4 chiral carbons in R. Did I do additional counting??


      There are only 2 chiral C atoms. But due to an internal plane of symmetry, the R,S and S,R stereoisomers are the meso isomer and thus optically inactive, and hence the only 2 enantiomeric optical isomers are S,S and R,R.

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    • Originally posted by Jh2424:

      Hello, I have another question regarding Ionisation Energy

      For noble gases, as to why they have the highest Ionisation Energy among the elements, is it because they are at the end of the period(where they have increased nuclear charge) or is it because they have a stable electronic configuration hence making extraction of electrons difficult

      Thanks :)


      The former, of course. The latter is not a valid explanation, merely a desperate attempt at 'smoking'.

      ____________________________________

      Originally posted by Jh2424:

      Hello, I have a question regarding melting points of HI vs HCl

      I know that melting point of HI is higher, is this due to the greater size of HI/number of electrons, less impact nuclear charge has on charge distribution hence greater polarizibility of electrons , leading to greater magnitude of partial charges on HI compared to HCl? 

      But the electronegativity difference of HCl is greater than HI, so what effect does this have on the strength of the Van der Waals forces?


      The magnitude of electronegativity difference, ie. polarity, has the opposite effect (ie. for the same molecular size and no. of electrons, the more polar the molecule, the stronger the intermolecular van der Waals interactions) but is strongly outweighed by the greater molecular size and no. of electrons, and hence polarizability of electron charge clouds and hence the magnitudes of partial charges and dipoles, hence strength of the van der Waals interactions, for HI over HCl. Consequently and concordantly, all factors considered, HI has a higher melting and boiling point compared to HCl.

      Originally posted by Jh2424:

      You mentioned 'the more polar the molecule, the stronger the intermolecular van der Waals interactions' Which van der Waals interactions are you talking about?( permanent dipole or instantaneous-induced dipole)

      And do you mean that you can only consider electronegativity difference if both molecules have same molecular size/electrons?


      Yes, of course.

      Anytime a polar molecule is involved, all 3 types of van der Waals interactions are relevant : Keesom, Debye, and London Dispersion forces.

      Originally posted by Jh2424:

      Sorry, i have another query regarding the abovementioned issue

      Then in the case where electronegativity does play a role ( both molecules are of same size) then what actually happens?(as in does partial charges get stronger or something else comes into play)


      Yes, of course. That's why (assuming similar molecular size) the more polar the molecule, the higher the melting & boiling points.

      _________________________

      Originally posted by Jh2424:

      Hello, would like to ask a question

      As to why c=c is stronger than c-c bond, is it because of the additional pi bond, or because the sigma bond in c=c is shorter due to increased S character in the sp2 hybridization causing the bond to be closer to the nucleus?


      Both, of course. In the A level exams, for any question, if you dunno which one Cambridge wants, always give both or all reasons.

      Edited by UltimaOnline 04 Dec `16, 10:55PM
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    • kyungsoostan and Sir3,

      A grade boundary for this year will most likely be approximately 75% (overall, combining all papers including SPA), depending on the entire cohort's performance.

      However, note that your own (ie. all students') estimation of your marks may be inaccurate (eg. you don't actually know your SPA marks, you may have made careless mistakes you didn't realize, Cambridge's required answer may be different from your school's taught answer, etc).

      Notwithstanding, assuming your estimation is accurate, then kyungsoostan's 77% is a safe A grade, and Sir3's 73.5% is a possible A grade. As long as you get > 70%, it's always *possible* to get an A grade (just that your chances increase dramatically at 75%), because the cohort's performance also has to be taken into account.

      ________________________________________

      Originally posted by GoatFacedAssassin:

      Guys I'm really trembling in fear... First i screw up paper2 (actually all my friends said it was hard) so i think i score around low B for that paper itself. Than for Paper 3, it was so easy! , but fml yeah i screw it up and my whole organic part was blank and i mixed up my R&C and haix. So my paper 3 was about low B too. yeah although i screw up, low B was quite possible. So based off p2 and p3 i got around 60-64%. Today's paper 2, i scored 27/40. First page all wrong idk what was i even thinking if not couldve 31 sia. but yeah its over. With those 3 papers i think i should be around 65 % AT MOST. Lets say spa affects my grades a little, will i drop to a D? I really do not want a H2 D... Haix. my uni course is AAB sia but i might end up with AAD if my chem really xiao suay.


      Prepare for the worst, hope for the best. From your description, your most likely grade is a C, with a small chance of B or D.

      Accordingly, be psychologically prepared to choose backup Uni courses with lower UAS requirements, and/or retake A levels as a private candidate (though if you have NS obligations, you've to decide exactly when to retake your A levels).

      http://www.nus.edu.sg/oam/gradeprofile/sprogramme-igp.html

      http://www3.ntu.edu.sg/oad2/website_files/IGP/NTU_IGP.pdf

      This may sound harsh, but it isn't meant to demoralize you (of course you would rather hear soothing assurances like "no worries, you'll probably still get your B grade" but that's mere consolation not honest clarification), but rather to remind you, that whatever happens, make the most of it. Use every opportunity in life to grow, including growing in fortitude and resilience. Doing badly for A levels isn't the end, there are always 2nd chances in life, but only if you're willing to give them to yourself.

      Edited by UltimaOnline 26 Nov `16, 12:32PM
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    • Originally posted by hoay:

      A MCQ from CIE June 2016, which i cannot reproduce here, gives a mass spectrum showing only ONE isotope of Copper having mass of 63.5 showing 100% abundance.

      The question was to prove which mass spectrum would give an Ar of 63.5. 

      The mass spectrum above would give 63.5 as the correct answer but this was not the answer. Another was the answer which different isotopes having different Whole number masses of isotope.

      My question is all isotopes of an element have Whole number masses. We have observaed this so far. So we can eliminate any choice having an isotopic mass in decimal....is this correct?

       


      Correct. The molar mass of any single isotope must be an integer (since there is no such thing as non-integer number of protons or neutrons), only averaged molar masses (ie. taking into consideration the relative abundance of various isotopes) can be non-integers.

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    • Do Singaporeans realize that burning joss papers and joss sticks generates airborne carcinogens, in the same way as smoking cigarettes causes cancer? It's up to you to follow your own religious beliefs, but it's bad ethics and bad karma to cause harm to others.

      Burning joss papers and joss sticks in HDB corridors is like forcing your neighbours to smoke carcinogenic cigarettes, and large scale burning during 7th month festivals is like forcing all other Singaporeans of other religions to smoke carcinogenic cigarettes.

      And here's the irony : no Chinese deity or Buddha ever instructed their followers to burn joss papers and joss sticks.

      It may be acceptable cultural practice back then, when there was no medical science to speak of, but today when medical science has clearly shown this activity's carcinogenic properties, you'd think humans would be intelligent enough and ethical enough to evolve their cultural practices to minimize harm to others and to themselves.

      2016 case : Sg man with China wife vs Singaporean aunties (joss paper smokers)http://theindependent.sg/video-of-prc-scolding-locals-for-burning-joss-papers-goes-viral/

      2015 case : Elderly Singaporean couple (joss sticks smokers) vs Young Singaporean couple with baby (helpless innocent victim)
      http://news.asiaone.com/news/singapore/caught-camera-neighbours-lash-out-each-other-over-joss-sticks

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    • Physicist Stephen Hawking got this right.

      The internet and the platforms that it makes possible allow very small groups of individuals to make enormous profits while employing very few people (eg. Facebook, Google, etc). This is inevitable, it is technological progress, but it is also socially destructive.

      We need to put this alongside the financial crash, which brought home to people that a very few individuals working in the financial sector (eg. Wall Street bankers) can accrue huge rewards and that the rest of us underwrite that success and pick up the bill when their greed leads us astray.

      So taken together we are living in a world of widening, not diminishing, financial inequality (ie. worsening Gini coefficient), in which many people can see not just their standard of living, but their ability to earn a living at all, disappearing. It is no wonder then that they are searching for a new deal, which Trump and Brexit might have appeared to represent.

      https://www.unlimited.world/unlimited/this-is-the-most-dangerous-time-for-our-planet

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    • H2 Chemistry

      Qn : What happens when you mix together solid carbon dioxide (−78.5 °C) and liquid nitrogen (−195.79 °C)? State the expected observations, expected temperature changes and/or changes in state for both species, giving the underlying chemical explanations.

      Ans : https://www.youtube.com/watch?v=mwpLg6-ZZcM

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    • Did they all fail their A level Chemistry? --____--

      This happened in Anhui’s Lu’An city in China. Birthday party-goers who were adjourning for the night got into a lift together with 20 balloons filled with hydrogen on Dec. 4.

      According to one of the victims, several people kept asking for the balloons to be lit, some even shouting for it. Despite protest from other lift-riders, light it they did.

      What should have been the popping of one balloon turned into a chain reaction of 20. The blast, described as deafening, was strong enough to cause the lift to apply its emergency brakes and the lift was stuck between the first and second floor. Wails were heard.

      Firefighters arrived to extract the trapped victims of idiocy. What they found were eight individuals with varying degrees of burns on their faces mixed with tears of regret.

      http://mothership.sg/2016/12/chinese-party-goers-felt-it-was-great-idea-to-light-up-20-hydrogen-filled-balloons-in-lift-bam-its-not/

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    • Originally posted by Jh2424:

      Hello, I have a question, how exactly does a non-polar solvent dissolve a non-polar solute?


      Whether the solute and solvent are polar or non-polar, the same consideration below applies. It's just that "like dissolves like" is a shortcut guideline that usually works.

      In terms of enthalpy change (delta H), heat released is comparable with heat absorbed (overall enthalpy change preferably exothermic, but can be slightly endothermic), in terms of physical interactions (van der Waals, and if applicable, hydrogen bonds or ionic bonds) formed and broken.

      In terms of entropy change (delta S), there will be a favourable positive entropy change upon mixing.

      Overall quantitatively, Gibbs free energy change must be negative for the dissolving to be thermodynamically favourable, according to the formula delta G = delta H - T x delta S.

      When you (ie. all students using this forum) ask me qns, and if you don't fully understand my replies, I won't spoonfeed you on the forum, so you'll have to research it further online (Google by thy sword, Wikipedia be thy shield), and/or ask your school teacher or private tutor, and/or join my BedokFunland JC tuition for more detailed, personalized guidance.


      Originally posted by Jh2424:

      So the 'energy' explaination for why something dissolves is more relevant than describing the physical process?( like how the polar water molecules pulls away the NaCl crystals)


      Of course, because the physical process is able to occur only because the overall thermodynamics is favourable enough to allow it.

      Edited by UltimaOnline 10 Dec `16, 9:04PM
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    • Originally posted by hoay:

      In order to calculate the most accurate mass of an ideal gas using the relationPV= nRT 

      what conditions of presuusre and temperatur would be used?

      Ans. We need mass to put into the final equation.....gases dont have masses they have volumes....so we have to liquefy them by applying high pressure and low temperature. Is this correct? 


      Gases do indeed have masses.

      The weights of gases are dependent on the gravitational force acting upon them, and difficult to measure directly, but the masses of gases are independent on the gravitational force acting upon them, and can be calculated using PV=nRT if simplified to ideal gas behavior, where P, V and T can be directly measured with appropriate apparatus, and sample mass = number of moles x molar mass. The mass of the gases obtained will be the weighted average of all gases present.


      Originally posted by hoay:

      Here is the full MCQ from CIE....

      Measured values of the pressure, volume and temperatureof a known mass of a gaseous compound are to be susbstituted into the equation

      Pv= nRT

       

      In order to calculate the relative molecular mass of the compound

      Which conditions of pressure and temperature would give the most accurate valuesof Mr?

                           pressure             temperature

      A                      high                     high

      B                      high                     low

      C                      low                     high

      D                      low                     low


      For this particular MCQ, the answer is C, because the behavior of gases approach ideal gas behavior most closely at high temperatures and low pressures. PV=nRT only holds true for ideal gases, not real gases.


      Originally posted by hoay:

      Okay. thank you.

      Can you through some light on the triplet point?

      Has this anything to do with the triplet point? 


      No, the "high temperature low pressure" for ideal gas behavior is a separate concept from the triple point of a species.

      A gas behaves more like an ideal gas at higher temperature and lower pressure, as the potential energy due to intermolecular forces becomes less significant compared with the particles' kinetic energy, and the size of the molecules becomes less significant compared to the empty space between them.

      Wikipedia (Uni level) : https://en.wikipedia.org/wiki/Ideal_gas
      A level syllabus relevance : http://www.chemguide.co.uk/physical/kt/realgases.html

      The triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.

      Wikipedia (Uni level) : https://en.wikipedia.org/wiki/Triple_point
      A level syllabus relevance : http://www.chemguide.co.uk/physical/phaseeqia/phasediags.html

      Edited by UltimaOnline 10 Dec `16, 10:15PM
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    • Originally posted by hoay:

      Given the strutucre of chloropyll what is correct

       

      1 dative covalency

      2 sigma bond

      3 sp3 hybridization

       

      1 and 2 are correct...but not sure about 3?? 

      Ohh....Is'nt due to dative and covalent bonding the shape around Mg is tetrahedral so sp3 hybridization ould be correct also....? Is this correct? 

       


      Not sp3, because the aromatic ring (either porphyrin or chlorin, depending on the exact chlorophyll type) is planar.


      Originally posted by hoay:

      okay.

      but what is he hybridization of Mg in this question? sp ?


      dsp2


      Originally posted by hoay:

      Now it is clear.

      What about Carbon atoms in hexagonal rings in graphite in each layer...they are co-planar.  


      More accurately periplanar rather than exactly coplanar, but Cambridge will accept coplanar.

      Edited by UltimaOnline 13 Dec `16, 4:01AM
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    • Originally posted by Jh2424:

      Hello, does water form 2 or 4 hydrogen bonds? Im not too sure about this, thanks


      Depends on state. As gaseous (assuming ideal gas behavior), 0. As liquid, fluctuates (many times per second) between 2 to 4. As solid, 4 (for the specific crystalline structure of ice relevant to A level syllabus).


      Originally posted by Jh2424:

      Thank you, I have another question, for ammonia, i dont understand why it forms just 2 hydrogen bonds, why can't the two hydrogens(left over after nitrogen and hydrogen) take part in hydrogen bonding?


      They can, but when you're talking about intermolecular H bonding between NH3 molecules, notice the ratio of H bond acceptors (ie. the N atoms) to H bond donors (ie. the H atoms) is 1:3, while the required ratio per H bond is 1:1.

      Analogously, in a class of 1 guy and 3 girls, how many (monogamic heterosexual) couples can you have at most? Concordantly, on average only 2 atoms (1 N and 1 H) can participate in H bonding between NH3 molecules.


      Originally posted by Jh2424:

      Sorry, i dont quite understand your explanation , going by your analogy, if 1 'guy' gets 1 'girl', whats going to stop the other 'girls' from getting other 'guys'? (unless you are talking about just 3 ammonia molecules)


      Then you'll run out of guys much sooner than girls, in a school of 10 classes, each class with 1 guy and 3 girls.

      Edited by UltimaOnline 17 Dec `16, 10:25PM
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    • Bone cancer survivor scores four distinctions
      http://www.straitstimes.com/singapore/education/bone-cancer-survivor-scores-four-distinctions

      To be realistic, the odds are stacked against her, because of her 'N' level academic background, and (this applies to all Singaporean students) because of the overly small number of places available in local medical schools. And if her medical condition does not resolve itself immediately or fully (only her oncologist will know her medical prognosis), it may continue to post further financial and time burdens that may distract her from performing her best along the way.

      However, should she indeed perform spectacularly well (ie. perfect scores for all subjects and all modules all the way here on out), her 'N' level academic background and her bone cancer medical condition, could actually help her stand out amongst the pool of candidates vying for a coveted place in medical school, as positive indications of her tenacity, perseverance and determination.

      Of course, the rich (especially ultra-rich) will always have multiple advantages over everyone else. If her family can afford it, then applying for overseas medical schools (of which there are thousands, though only a few dozen or so are recognized for practicing medicine in Singapore), in addition to local medical schools, will naturally increase her chances of fulfilling her career ambitions. And if you're rich enough to locate yourself overseas permanently, you could even go for medical schools not recognized for practicing medicine in Singapore, as a last resort.

      I wish her all the best in her health and in her medical doctor career ambitions.

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    • Originally posted by hoay:

      CO2 has same melting and boiling point. Is is due to the structure and non-polar nature?


      Yes, the fact that CO2 is overall non-polar, yet possesses 2 significant individual dipoles in a linear molecular geometry, results in its sublimative property (ie. only existing at solid and gaseous states) at 1 bar or 1 atm. The liquid state is only possible at significantly higher pressures, as illustrated in its phase diagram.

      image

      Image credits : Wikipedia

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    • Drinkable ethanolic drinks (eg. beer, wine, liquor, etc) are taxed heavily by governments (a considerable source of money for governments because who doesn't like an ethanolic drink?), so unethical and unscrupulous importers, distributors or retailers sometimes adulterate their ethanolic drinks with cheap, untaxed methanol to sell to unsuspecting customers. In many cases, most of the victims go permanently blind as a result of methanol poisoning, if they manage to survive at all.

      Toxic liquor kills 24 in Pakistan
      https://sg.news.yahoo.com/toxic-liquor-kills-21-pakistan-police-051343831.html

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