Miscellaneous Chemistry Problems from 2004 Australian NQE

1. Sep 1, 2007

TripleS

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. Sep 2, 2007

Astronuc

Staff Emeritus
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.