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2017 H2 Chemistry JC1 & 2 students post your questions here

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  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    14,097 posts since May '05
  • Carychidestar's Avatar
    14 posts since Jan '17
    • Hi ultima,how do you solve this question?

      "A 0.01 mol sample of an oxochloride of vanadium,VOCly, required 20.00cm3 of 0.1mol dm3 acidified potassium manganate(VII) solution for oxidation of the vanadium to the +5 oxidation state"

      Find the value of y"

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    14,097 posts since May '05
    • Originally posted by Carychidestar:

      Hi ultima,how do you solve this question?

      "A 0.01 mol sample of an oxochloride of vanadium,VOCly, required 20.00cm3 of 0.1mol dm3 acidified potassium manganate(VII) solution for oxidation of the vanadium to the +5 oxidation state"

      Find the value of y"


      Hi Carychidestar,

      Find OS of V (in VOCly) in terms of y.

      Find moles of MnO4- used.

      Find total moles of electrons transferred based on stoichiometry.

      Find moles of electrons transferred per mole of V.

      Initial OS (in terms of y) + moles of electrons transferred per mole of V = Final OS (ie. +5)

      Solve for y.

      Edited by UltimaOnline 16 Apr `17, 4:35PM
    • "If you train early in the day, Ms Bernardes recommends water with a pinch of sea salt when you wake up, to replenish the salts you lose breathing throughout the night."

      Source :
      http://www.straitstimes.com/world/the-trouble-with-setting-a-blistering-pace
      http://www.oanow.com/news/local/training-tips-for-new-runners-beware-raw-thighs-tummy-troubles/article_ea92c190-206d-11e7-b461-03037cf8dccd.html

      What nonsense! Now I don't doubt that Debi Bernardes is an expert triathlon coach, but she seems to have failed her H2 Chemistry, to make such a ridiculous error in her statement. If Cambridge asks, you do know how to explain what her conceptual error is, don't you?

    • A classic question that is now making the rounds in Singapore JCs' tutorials again. Have fun with it. As usual, BedokFunland JC students can ask me how to solve it during tuition, all other students can go ask their school teacher or private tutor.

      Xenon and fluorine were mixed in a 1:1 ratio by volume and sealed in a vessel attached to a barometer. After some time, crystals were found to have formed on the walls of the vessel and the barometer showed that the pressure had dropped to 70% of the original pressure. Analysis of the gases remaining indicated 4 parts xenon to 3 parts fluorine. Construct a balanced equation for the reaction. Assume constant temperature throughout.

    • image

    • May 2017 Special Conditional Offer for JC2 Students who are interested in H2 Chemistry tuition.

      If you cannot afford tuition, feel free to (make a SgForums account) and post your H2 Chem questions here, I'll help you out for free here.

      Edited by UltimaOnline 16 May `17, 8:08PM
  • Carychidestar's Avatar
    14 posts since Jan '17
    • Hi ultima,how do you solve these two questions?

      '50cm3 of a 0.1 mol dm3 of a solution of a metallic salt reacted exactly with 25cm3 of sulphite solution.The half equation for the oxidation of sulphite ion is 

      SO3^2- + H2O -> SO4^2- + 2H^+ +2e

      If the original exidation number of the metal in the salt was 3, calculate the new oxidation number of the metal'

      I only managed to find the no.moles of the salt and I'm stucked.

      'Half-equations

      2CO2 + 2e -> C2O4^2-

      FE^3+ + e -> FE^2+

      MNO4^- + 8H^+ + 5e -> MN^2+ + 4H2O

      Calculate the no.moles of MNO4^- ion that will react with 1 mol of iron(II) ethanedioate;FeC2O4 in acidic solution'

      This one,I dont even know where to start since there are no numbers provided.

      Edited by Carychidestar 23 Apr `17, 6:09PM
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    14,097 posts since May '05
    • Hi Carychidestar, do you want to consider joining my BedokFunland JC tuition? Personalized guidance will help you a lot more effectively than can be done through an online forum. You can PM me if you're interested.

      1st qn : qn is flawed becoz the counter-anion is not specified (so let's assume it's uninegative), and the molarity of sulphite solution not given (so let's assume it's also 0.1M). 2 mol of M3+ reacts with 1 mol of SO3 2-, which when oxidized to SO4 2-, releases 2 mol of e-. Hence 2 mol of M3+ accepts 2 mol of e-, ie. final OS of metal = +2.

      2nd qn : each mol of FeC2O4 releases 3 mol of e- upon oxidation (as each Fe2+ is oxidized to Fe3+, and each C2O4 2- is oxidized to 2 CO2). Each mol of MnO4- accepts 5 mol of e- upon reduction in acidic pH. Hence each mol of FeC2O4 will react with 3/5 mol of MnO4-.

      Originally posted by Carychidestar:

      Hi ultima,how do you solve these two questions?

      '50cm3 of a 0.1 mol dm3 of a solution of a metallic salt reacted exactly with 25cm3 of sulphite solution.The half equation for the oxidation of sulphite ion is 

      SO3^2- + H2O -> SO4^2- + 2H^+ +2e

      If the original exidation number of the metal in the salt was 3, calculate the new oxidation number of the metal'

      I only managed to find the no.moles of the salt and I'm stucked.

      'Half-equations

      2CO2 + 2e -> C2O4^2-

      FE^3+ + e -> FE^2+

      MNO4^- + 8H^+ + 5e -> MN^2+ + 4H2O

      Calculate the no.moles of MNO4^- ion that will react with 1 mol of iron(II) ethanedioate;FeC2O4 in acidic solution'

      This one,I dont even know where to start since there are no numbers provided.


      Edited by UltimaOnline 23 Apr `17, 8:58PM
    • _

      Edited by UltimaOnline 13 Oct `17, 2:38AM
  • flowerd's Avatar
    5 posts since Mar '17
    • what is the difference between heat and heat under reflux for organic chemistry conditions? for example, is it acceptable to write "heat" instead of "heat under reflux" when converting halogenoalkane to nitrile?

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    14,097 posts since May '05
    • Originally posted by flowerd:

      what is the difference between heat and heat under reflux for organic chemistry conditions? for example, is it acceptable to write "heat" instead of "heat under reflux" when converting halogenoalkane to nitrile?


      While in many instances, Cambridge will accept either term (unless of course, "warm" or "heat with distillation" is specifically required for some reactions), however as a general rule, it is advised that students write "heat" for distinguishing chemical tests, and "heat under reflux" for organic synthesis pathways.

      Cambridge can ask, and in the past, have indeed asked, questions on drawing the reflux apparatus, and on when or why reflux is preferred or required in certain practical setups. For this, the following Wikipedia page is useful for students.

      image

      https://en.wikipedia.org/wiki/Reflux#Reflux_to_regulate_energy_in_chemical_reactions

    • _

      Edited by UltimaOnline 13 Oct `17, 2:39AM
    • Acid-Base Equilibria remains one of the toughest topics for many H2 Chem students. Here is an excellent 2016 NYJC Qn for your practice.

      20.0cm3 of an unidentified weak monoprotic Bronsted-Lowry base of unknown molarity was titrated with 0.01M of a strong monoprotic Bronsted-Lowry acid at 25 degrees C. The initial pH at the start of the titration was 11.0, and equivalence point was reached when 25.0cm3 of acid was added. Calculate the Kb of the weak base, as well as the pH of the solution when 35.0cm3 of acid was added.

  • qixx's Avatar
    24 posts since Jan '17
    • Can you give a crash course on what lattice energy is about and what are some possible questions that can be tested relation to lattice energy? Also, is lattice energy & ionisation energy the same ?

       

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    14,097 posts since May '05
    • Originally posted by qixx:

      Can you give a crash course on what lattice energy is about and what are some possible questions that can be tested relation to lattice energy? Also, is lattice energy & ionisation energy the same ?

       


      Sounds like you could really use some help from tuition. Consider BedokFunland JC for your H2 Chem tuition needs.

      For A level Chem crash courses, the most widely used website on the internet is (retired A level examiner) Jim Clark's ChemGuide website :

      http://www.chemguide.co.uk/

      No, lattice energy and ionization energy have different definitions, both of which you're required to memorize. You can easily Google or Wikipedia these definitions for yourself.

      Reminder to all : for crash courses or definitions or factual info, please Google or Wikipedia out these for yourself. I'll only guide you, not spoonfeed you, nor do your homework for you.

    • Here's a BedokFunland JC example of a possible A level exam question which includes (but not limited to) lattice energy. I won't reveal the answer here, but if you (ie. JC students) wanna try posting your attempted answer here, I'll let you know if you got it correct. Students can go ask their school teacher or private tutor.

      Explain why (under standard conditions), when magnesium and chlorine reacts, the formation of MgCl2(s) is thermodynamically favored over the formation of either MgCl(s) or MgCl3(s).

      Edited by UltimaOnline 13 Oct `17, 2:39AM
    • A BedokFunland JC H2 Chemistry Question

      Explain how radiocarbon dating works for determining the age of an object containing organic material.

    • _

      Edited by UltimaOnline 13 Oct `17, 2:37AM
    • _

      Edited by UltimaOnline 13 Oct `17, 2:38AM
  • flowerd's Avatar
    5 posts since Mar '17
    • when writing the name of mechanism, do i write SN1/SN2 or just nucleophilic substitution? 

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    14,097 posts since May '05
    • Originally posted by flowerd:

      when writing the name of mechanism, do i write SN1/SN2 or just nucleophilic substitution? 


      Both. When in doubt, always write everything you can think of.

    • _

      Edited by UltimaOnline 13 Oct `17, 2:36AM
  • glitter58's Avatar
    18 posts since Mar '17
    • when do i need to write state symbols for chemical equations? for example in the topic of periodicity, do i have to write state symbols even when the qn doesnt require me to? 

    • for ionic eqns, is it a must to write state symbols?

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