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    • Originally posted by hoay:

      Thank you for the detailed answer.


      Totally welcome, feel free to keep your CIE questions coming! :)

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    • Originally posted by hoay:

      The following is aquestion from CIE A-level (9701) Nov 2010 

      G is NH2(CH2)3NH2 is titrated with HCl(aq).

      A 0.10 mol dm–3 solution of G has a pH of 11.3. When 30 cm3 of 0.10 mol dm–3 HCl is added to 10 cm3 of a 0.10 mol dm–3 solution of G, the final pH is 1.6.

      Using the following axes, sketch the pH changes that occur during this addition of

      HCl(aq). [I cannot show the axes here] 

       

      No other data such as pka values were given. Its was a 2 marks question.

      Solution :  I only knew the starting pH that is 11.3 and the end pH 1.6. There will be two humps as the base is diacidic. The end-point i have no idea. Please help. 


      Here is my BedokFunland JC answer to this 2010 Cambridge A level Exam Qn.

      Since molarities of both acid and base are the same, the volumes of HCl required for 1st and 2nd equivalent points are 10cm3 and 20cm3.

      Show a steeper and longer vertical portion for the 1st equivalence point, with slightly acidic pH, eg pH 6.

      Show a less steep and shorter vertical portion for the 2nd equivalence point, with a strongly acidic pH. eg pH 2.

      Rationale :

      Although technically diprotic, but you can deduce that it in practice it is mostly monoprotic (ie. extremely small Kb2), because of 3 (inter-related) reasons :

      1) Considering the unipositive conjugate acid, the unipositive formal charged N atom / unipositive BH+ ion, electrostatically repels the H+ / H3O+ cation. Hence kb2 would be extremely small.

      2) Considering the unipositive conjugate acid, the unipositive formal charged N atom is electon-withdrawing by induction, making the lone pair on the other N atom (3 C atoms away) to be less available to accept protons. Hence kb2 would be extremely small.

      3) Considering the dipositive conjugate acid, 2 positively formal charged N atoms with only 3 C atoms between them is highly destabilizing, due to inter-nuclei repulsions within the highly unstable high cationic charge density dipositive conjugate acid. The more unstable the conjugate acid, the weaker the conjugate base. Hence kb2 would be extremely small.

      As such, regard 1,3-diaminopropane as mostly monoprotic, in which case you can (even without Kb values available) expect the 1st equivalence point to have a acidic pH (due to proton dissociation hydrolysis of the acidic ammonium salt).

      Which is why we draw a steeper, longer vertical portion for the 1st equivalence point, and a less steep, shorter vertical portion for the 2nd equivalence point (since Kb2 will be extremely small).

      As for the strongly acidic pH at the 2nd equivalence point, as explained earlier, Kb2 is extremely small, meaning Ka1 (ie. proton dissociation for the dipositive conjugate acid) is extremely large, meaning that the dipositive conjugate acid is a strong acid. Hence at 2nd equivalence point, the dipositive diammonium salt is strongly acidic and will have a pH almost as acidic as the final pH of 1.6 (ie. with 10cm3 excess HCl). A reasonable estimate would be approx pH 2.



      Originally posted by hoay:

      Thank you for the detailed answer.

      Is there any book you would recommend for these type of pian-staking problems?

       


      If you're referring specifically to acid-base equilibria problems involving polyprotic / multiprotic acids & bases, there's plenty of free materials available on the internet :
      https://www.google.com.sg/#q=polyprotic+acid+base+equilibrium+problems

      Edited by UltimaOnline 23 Dec `14, 4:07AM
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    • Originally posted by hoay:

      The following is aquestion from CIE A-level (9701) Nov 2010 

      G is NH2(CH2)3NH2 is titrated with HCl(aq).

      A 0.10 mol dm–3 solution of G has a pH of 11.3. When 30 cm3 of 0.10 mol dm–3 HCl is added to 10 cm3 of a 0.10 mol dm–3 solution of G, the final pH is 1.6.

      Using the following axes, sketch the pH changes that occur during this addition of

      HCl(aq). [I cannot show the axes here] 

       

      No other data such as pka values were given. Its was a 2 marks question.

      Solution :  I only knew the starting pH that is 11.3 and the end pH 1.6. There will be two humps as the base is diacidic. The end-point i have no idea. Please help. 


      Here is my BedokFunland JC answer to this 2010 Cambridge A level Exam Qn.

      Since molarities of both acid and base are the same, the volumes of HCl required for 1st and 2nd equivalent points are 10cm3 and 20cm3.

      Show a steeper and longer vertical portion for the 1st equivalence point, with slightly acidic pH, eg pH 6.

      Show a less steep and shorter vertical portion for the 2nd equivalence point, with a strongly acidic pH. eg pH 2.

      Rationale :

      Although technically diprotic, but you can deduce that it in practice it is mostly monoprotic (ie. extremely small Kb2), because of 3 (inter-related) reasons :

      1) Considering the unipositive conjugate acid, the unipositive formal charged N atom / unipositive BH+ ion, electrostatically repels the H+ / H3O+ cation. Hence kb2 would be extremely small.

      2) Considering the unipositive conjugate acid, the unipositive formal charged N atom is electon-withdrawing by induction, making the lone pair on the other N atom (3 C atoms away) to be less available to accept protons. Hence kb2 would be extremely small.

      3) Considering the dipositive conjugate acid, 2 positively formal charged N atoms with only 3 C atoms between them is highly destabilizing, due to inter-nuclei repulsions within the highly unstable high cationic charge density dipositive conjugate acid. The more unstable the conjugate acid, the weaker the conjugate base. Hence kb2 would be extremely small.

      As such, regard 1,3-diaminopropane as mostly monoprotic, in which case you can (even without Kb values available) expect the 1st equivalence point to have a acidic pH (due to proton dissociation hydrolysis of the acidic ammonium salt).

      Which is why we draw a steeper, longer vertical portion for the 1st equivalence point, and a less steep, shorter vertical portion for the 2nd equivalence point (since Kb2 will be extremely small).

      As for the strongly acidic pH at the 2nd equivalence point, as explained earlier, Kb2 is extremely small, meaning Ka1 (ie. proton dissociation for the dipositive conjugate acid) is extremely large, meaning that the dipositive conjugate acid is a strong acid. Hence at 2nd equivalence point, the dipositive diammonium salt is strongly acidic and will have a pH almost as acidic as the final pH of 1.6 (ie. with 10cm3 excess HCl). A reasonable estimate would be approx pH 2.

      Edited by UltimaOnline 23 Dec `14, 3:44AM
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    • Man attacks woman with nail clipper for reporting his sexual assault on drunk girl to the police

      image

      Stomper Suganthi was threatened with a nail clipper for confronting two men who behaved inappropriately with a woman.

      The incident happened at Block 406 Admiralty Link around 8pm yesterday (Dec 21).

      According to the Stomper, the woman's dress had risen all the way above her navel and one of the men had his hand in her panties.

      She said:

      "The woman was extremely drunk and the two men were tipsy.

      "The woman's undergarments were exposed and the men were touching the woman everywhere."

      She added that their inappropriate behaviour attracted lots of attention from passersby but they continued with their actions despite small children being around.

      Suganthi then went ahead to confront the two men but they started shouting at her.

      The Police was then called.

      The Police arrived on the scene and called in a female companion of the woman to bring her home.

      They then talked to the two men and all of them left the scene within half an hour.

      But, Stomper Suganthi was still at the scene and was sitting at the void deck with her husband.

      Moments later, one of the men came back, this time holding a nail clipper and tried to attack Suganthi.

      The Police was then called in again.

      http://singaporeseen.stomp.com.sg/si...n-for-behaving

      Edited by UltimaOnline 22 Dec `14, 4:01PM
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    • image

      Shannon Perry Ignatius posted on Facebook :
      https://www.facebook.com/shannon.aka.bumblebee

      December 18 at 8:54pm

      Thank You for reading and forwarding this.

      Dear All,

      I would like to appeal to all of you to spread this message to everyone so that I can find out who was the driver of the Off White Plain Van that hit me and ran away. I was riding a black Yamaha bike. It was a hit and run.

      It happened on the SLE before Exit 11 towards BKE(KJE/PIE) Turf Club Ave / Mandai Road. At around 1145am to 1152am on the 17th Dec 2014

      It was a 3 Lane carriageway. I was changing lanes from lane 2 lane 3 when the Van hit me on my right. The van was in front of me in Lane 2, and when I changed to lane 3 the Van was on my right.

      I suspect the van driver did not check his blind spot when changing to Lane 3 and that was when he ram into my bike. My bike was wobbling badly as it was a very hard ram with a “boom” sound.

      After the hard knock on my right, the van drove off immediately as quickly as possible in Lane 2.

      At that point of time I knew my right hand was already injured as I could not press on my brakes. I tried my best to stabilize the bike and I stopped at the road shoulder.

      When I got off my bike I knew my right leg and my right hand was already injured. I than called for the police and ambulance immediately and they arrived 20 minutes later.

      The police came and I gave my statement in the ambulance. I was then sent to Khoo Teck Puat Hospital.

      I had to undergo surgery as the laceration on my last finger was too deep and I could see my bone. After taking the x-ray, the doctor told me I had a fractured index finger and my last finger muscle tendon was torn and I had to undergo surgery. My right knee will also swollen with a hematoma.

      Any vehicles or car behind me with a video camera please get back to me or pass the information to the Police ASAP.
      — feeling hopeful.

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    • Originally posted by hoay:

      The boiling point of CCl4 is higher than SiCl4. CCl4 has Vander waal forces as well as SiCl4 has but the b.p of SiCl4 is expected to be higher than CCl4 as the former has more spacious electron cloud. Can someone explain. 


      I actually posted the answer to this question on my BedokFunland JC website several years ago :

      Due to the greater magnitude of electronegativity difference between Si and Cl, compared to C and Cl, there is a larger magnitude of partial negative charge on the Cl atoms in SiCl4 molecules (compared to CCl4 molecules), and consequently a greater extent of electrostatic van der Waals repulsions between SiCl4 molecules, and therefore SiCl4 has a lower boiling point, compared to CCl4.

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    • Originally posted by hoay:

      The boiling point of CCl4 is higher than SiCl4. CCl4 has Vander waal forces as well as SiCl4 has but the b.p of SiCl4 is expected to be higher than CCl4 as the former has more spacious electron cloud. Can someone explain. 


      I actually posted the answer to this question on my BedokFunland JC website several years ago :

      Due to the greater magnitude of electronegativity difference between Si and Cl, compared to C and Cl, there is a larger magnitude of partial negative charge on the Cl atoms in SiCl4 molecules (compared to CCl4 molecules), and consequently a greater extent of electrostatic van der Waals repulsions between SiCl4 molecules, and therefore SiCl4 has a lower boiling point, compared to CCl4.

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    • http://therealsingapore.com/content/returning-singaporean-shocked-how-much-singapore-overrun-fts

      Was recently back in Singapore after being based overseas for many years. Chanced upon and subscribed to the EDMW forum when I was abroad and read many rants about the Government’s ‘Foreign Talent’ policies.

      Felt then that people were complaining for the sake of complaining. Could not reconcile with all the anger and frustration until I landed back in my beloved Singapore. The online scenario painted by many an exasperated netizens held much truth.

      Everywhere on the island is now much more crowded than before. Uncouth and anti-social behaviours by foreigners are more evident. The general level of cleanliness around the city has also dropped. General service standard and productivity have also dropped noticeably.

      Beyond the physical deterioration, I have also observed a disturbing trend emerging. Singaporeans are being pushed out of the labour market by unfair practices across all job-levels. Blue-collared workers have their wages depressed and in some cases, even employment denied because the foreigner foreman chose his own countrymen over a deserving Singaporean worker. Entry and mid-level white-collared workers are denied deserving job-promotions and career-advancement opportunities because of unfair advantages that their foreigners colleagues enjoyed.

      Even top level executive-positions are not spared as foreigner bosses/HRs/CEOs choose to bring in their own kind and/or network instead of hiring/promoting a deserving and in many instances, better-qualified Singaporeans. Heard plenty of stories from friends and head-hunters. A large oil & gas firm bringing in foreigners to fill regional management positions that capable local Singaporean managers could and would want to do. A Swiss bank parachuting executives from Zurich and Geneva into Singapore to fill management roles yearned by the local employees because the market in Europe is depressing now. A French company choosing a 60-year old French citizen with a mediocre track-record ahead of a top-performing Singaporean in his mid-40s to be the regional CEO.

      Having lived and worked in different global cities, I have seen my fair share of discrimination in the labour market. American banks and financial institutions on TARP not allowed to employ non-Americans, companies in Europe are incentivised to only employ EU citizens, international banks in Japan only employs Japanese natives and even fluent-Japanese speakers are denied the opportunity to interview, only native-Spanish speakers will ever be employed in Latin America etc. These discriminatory labour practices that favour the natives are not unexpected. After all, it’s their country and their rules. But, seeing deserving Singaporeans being discriminated in his/her own country is really shocking to say the least.

      Singapore has always made meritocracy as one of our main pillars for nation-building and progress. It’s a real shame that this value is being eroded by the relentless influx of foreigners, who valued kinship and people of their own skin-colour more. The Government, with its liberal foreign employment policies, knowingly or otherwise, become the perfect accomplice. This even despite the massive outcry from the populace during the GE2011, that prompted the Prime Minister to apologize on national television no less.

      Alas, words are not backed up by actions. Worse, hard figures published recently showed the contrary as more foreigners are allowed to work in Singapore in the past year since GE2011. In Leadership 101, what is worse than incompetency is insincerity and betrayal. It’s no wonder that this current Government is losing its credibility and whatever residual goodwill that 60.1% of the populace has decided to give it a last-chance to redeem itself.

      - Government has always emphasized that we need more foreigners for the country to stay competitive and achieve higher GDP growth. Yes, the Singaporean economy has stayed very competitive globally and our GDP at US$56,532 per capita is the highest in the world, (US$5,300 more than second-placed Norway!) according The Wealth Report 2012. Several questions to ask. How have you personally benefited from all these economic progress? And is this US$56,532 the median income for Singaporeans? If not, why is the Singaporean’s median income at US$29,500 so much lesser than the mean GDP figure?

      - Economic progress and high GDP growth have a very low correlation with the quality of life of the populace. So why does the Government keep on insisting on maintaining high GDP growth? Is the Government’s raison d’être (reason for existence) to improve the people’s quality of life or to achieve high GDP growth? Which is the mean and which is the end? So really who benefits from the high GDP growth?

      - Government keeps on emphasizing that foreigners are needed to safe keep jobs for Singaporeans. Then why is that the case that 7 out of 10 jobs created in 2011 goes to a foreigner?

      - Government keeps on saying that Singaporeans do not possess the necessary skill-sets for certain jobs that MNCs in Singapore need, so foreigners need to be imported. Ask the critical question that if we keep on importing foreigners, then how can Singaporeans ever be trained to have these skill-sets? And this argument become circular the next time round in the future. Since the MNCs do not take in and train local Singaporeans in these skill-sets, it’ll forever use the lack of necessary skill-sets excuse to keep on demanding for the Government to allow more foreigners in.

      - The official unemployment figure is 2%. How is this figure calculated? What is the under-employment figure for Singaporeans? What is the real cost to the nation of educating Singaporeans to such high international standards and have him/her under-employed because of unfair labour practice promulgated by foreigner bossess/HRs/CEOs?

      - Why did the Government allowed in an addition of 1.5 million foreigners without adequate preparation of the transport and housing infrastructure? Complacency or incompetency? What’s is the real cost to the country with such high housing cost? If people are spending a high proportion of their income on housing, the amount of disposable income left for other consumption and retirement planning would suffer. So what’s the ultimate cost to the country in the long-term? And why is S$1.1 billion Singaporean tax-dollars used to increase the nation’s bus capacity, subsidizing the 40% non-Singaporeans in the process?

      - And the million dollar question (no puns intended). Have your life become better or worse off since the last GE?

      http://therealsingapore.com/content/returning-singaporean-shocked-how-much-singapore-overrun-fts

      Edited by UltimaOnline 20 Dec `14, 4:00AM
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    • Top 10 Creepy PC / Video game theories.

      https://www.youtube.com/watch?feature=player_embedded&v=dDUsG_MZNi4

      "Top #1 Creepy game theory : The game (in which you can choose to play either a girl trying to survive, or an inter-dimensional demon) self-deletes when you die or when complete the game. A few years ago, a Japanese man paid US$700,000 on ebay for the last remaining copy, and announced that he would upload to YouTube a walkthrough video of himself completing the entire game. But in the end the only video he uploaded was himself staring at his PC monitor and crying. Creepy much?"

      Dude, if I just lost US$700,000 when the game self-deleted, I would also be crying.

      Edited by UltimaOnline 18 Dec `14, 8:34PM
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