Miss Martoman's questions

[Martoman]

Jumping on the bandwagon here.

Some of these will appear incredibly stupid  

1. all gas. Measured at the same pressure and temp, if 40L of ammonia is mixed with 70 L of the final volume of the gas mixture will be?

I say it reacts in its ratios so its just 40 + 60 = 100?

2. How does an aliquot of anything have the same concentration? Aren't we changing the volume and mole so that means the concentration WILL change?

3 10g of is obtained when combusting in a limited supply of oxygen, what is the percentage yeild of carbon dioxide?

If in a limited supply I didn't think would even form?

[fady_22]

1: Yep, that's right.
2: I don't really understand. If you mean taking an aliquot of a solution, the concentration changes, then you are wrong. The concentration, remember, is the amount of something (usually mole) per litre. You ARE changing the volume, but proportionally changing the amount of mole so that the concentration remains constant.
3: What you do is find the theoretical yield of carbon dioxide(I'm assuming you are given the mass of methane that is reacting)-- this is the theoretical yield of the reaction, assuming that it went to completion. This would be different to the 10 g of CO2 actually obtained, so find what percentage this actual yield is of the theoretical yield. Also one word of advice: don't overcomplicate things. Assume that the combustion was complete and carbon dioxide formed.

[Martoman]

*cries* answer for 1) is 120 L.

The mass of methane is 16 g here. but it says limited supply??

[fady_22]

*cries* answer for 1) is 120 L.

The mass of methane is 16 g here. but it says limited supply??

Exactly, it says limited supply. So the percentage yield cannot be 100%, now can it?

If the mass of methane is 16g:

The equation is (assuming complete combustion):
CH4+2O2----->CO2+2H2O
Therefore, n(CO2)=1 mol and m(CO2)= 44 g

[stonecold]

wtf question 1!  where does 120L come from...

[kyzoo]

Omg I know I've seen Q1 before, where it is from =/

[Martoman]

TSFX. I think they be on crack.

[kenhung123]

It should be 110 isn't it?simply just add?

[stonecold]

stupid tsfx bastards...

[Greggler]

the tsfx exams are not even worth doing. they rush them out a few days before the exam every year and are always so irrelevant and dodgy

[kenhung123]

Thanks for the advice, I will throw them in the bin now...(damn waste of 20 pages)

[stonecold]

i know! i printed like 7 of those exams for chem and bio.

poor trees

[Martoman]

‘Rubber’ is used in numerous applications, ranging from simple ‘elastic’ bands to tyres for
motor vehicles and aircraft. SBR, a synthetic rubber, is a copolymer of styrene, C8H8, and
butadiene, C4H6, combined by an addition reaction in a ratio that produces a product with an
empirical formula of C10H13.
In what mole ratio are styrene and butadiene combined to produce SBR?

I just don't get this at all.

[stonecold]

It is 1:3. Don't ask me why/how, I just did trial and error haha...

1 x styrene = C8H8  +  3 x butadiene (C4H6) = C12H18  

=C20H26

Then the simplest ratio is C10H13

[Martoman]

Haha yes this is what I did.... but i don't really understand it either