1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Miscellaneous Chemistry Problems from 2004 Australian NQE

  1. Sep 1, 2007 #1
    Question 1:
    1. The problem statement, all variables and given/known data
    In the early years of the nineteenth century, John published a list of atomic masses. One experiment on which it was based involved measuring the composition by mass of ammonia. By assigning hydrogen a mass of 1 unit and using the chemical formula for ammonia, he could deduce the atomic mass of nitrogen.

    Unfortunately, he incorrectly believed that the formula for ammonia was NH. What atomic
    mass did he propose for nitrogen on the basis of this formula?

    2. Relevant equations
    None?

    3. The attempt at a solution
    Molar Mass of NH3 = 17
    Thinking H is 1, and the formula is NH then
    N = 16? (Except this is wrong...)

    Question 2
    1. The problem statement, all variables and given/known data
    A brown-black compound of thallium was found to contain 89.5% Tl and 10.5% O. What is
    the oxidation number of thallium in this compound?

    2. Relevant equations
    None again?

    3. The attempt at a solution
    204.4 / 89.5 = 2.28
    16 / 10.5 = 1.52
    2.28 x 2 = 4.5 ish
    1.52 x 3 = 4.5 ish

    so 3 oxygens to 2 thalliums...so 3+
    but im pretty sure thats not how you do it...its the right answer though...

    Question 3
    1. The problem statement, all variables and given/known data
    A sample of phosphorus pentachloride is placed in a sealed container, where it decomposes into phosphorus trichloride and chlorine gas.

    Once the mixture is at equilibrium, a small quantity of helium gas is added, at constant pressure and temperature, and the mixture is allowed to return to equilibrium. Which of the following best describes the second equilibrium position with respect to the first?

    A - The second equilibrium position is the same as the first, because helium is much
    lighter than any of the other molecules in the container and will have a negligible
    effect on the reaction.
    B - The second equilibrium position is the same as the first, because helium does not
    react with any of the other molecules in the container.
    C - The second equilibrium position has more PCl3 than the first.
    D - The second equilibrium position has more PCl5 than the first.
    E - It is impossible to tell what will happen without knowing the equilibrium constant
    for this reaction.

    2. Relevant equations
    PCl5 (g) <---> PCl3 (g) + Cl2 (g)

    3. The attempt at a solution
    Addition of helium would increase the number of gas particles and the system would then react to the left in order to decrease the increased pressure.
    So I would choose D, except the answer was C

    Question 4:
    1. The problem statement, all variables and given/known data
    Q14 An unknown compound was discovered to have molecular formula C10H20O2. To which of the following classes of compounds could this unknown not belong?

    A - alkynes
    B - alkenes
    C - alcohols
    D - esters
    E - ethers

    2. Relevant equations
    Alkynes = CnH2n-2
    Alkenes = CnH2n

    3. The attempt at a solution
    It could be a decene-diol, so A and C would work.
    And i wasn't sure about the rest (I wasn't sure what a ester or ether was ><)
     
  2. jcsd
  3. Sep 2, 2007 #2

    Astronuc

    User Avatar

    Staff: Mentor

    1. Remember in NH3, so the actual masses are 14 (N) and 3 (H), but John (Dalton, I presume to one is referring) used the formula NH. The masses should be 14 + 3 = 17, but changing the basis by dividing by 3 to make mass of 3H to be 1 would give 14/3 + 3/3 = 17/3.

    So three H was assigned 1, and the remaining mass 14 would be assigned 14/3.

    http://en.wikipedia.org/wiki/John_Dalton

    2. For the Tl oxide compound, assume a given mass, e.g. 100 g, which would give 89.5 g of Tl and 10.5 g of O. Divide each quantity by the respective atomic mass, which yields the number of moles, and then find the formula that gives both values of moles as integers.

    3. I would expect the He to tend to increase pressure and push the equilibrium to favor more PCl5 - which occupies less volume.

    4. http://en.wikipedia.org/wiki/Organic_compound (See ester)

    Perhaps chemistree or other chemistry guru will drop in to assist.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Miscellaneous Chemistry Problems from 2004 Australian NQE
  1. Chemistry problem (Replies: 1)

  2. A chemistry problem (Replies: 4)

  3. How many moles of ATP (Replies: 3)

  4. Chemistry problem (Replies: 9)

  5. Chemistry problem (Replies: 1)

Loading...