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

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    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
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    11,434 posts since May '05
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    11,434 posts since May '05
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    11,434 posts since May '05
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    11,434 posts since May '05
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    11,434 posts since May '05
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    11,434 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, 4:36PM
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    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
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    11,434 posts since May '05
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    11,434 posts since May '05
    • 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.

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    11,434 posts since May '05
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    11,434 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
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    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
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    • This question can actually be done at O levels, without A level knowledge, if you're smart enough. Whether you're a O level or A level student, time yourself to see how long you take to solve this question.

      image

      Pssst... 2017 JC2 students be sure to check out my limited-time offer before it expires... clock is ticking... 6 months left to A level exams...

      Edited by UltimaOnline 25 Apr `17, 3:15AM
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    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
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    • 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

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    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
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