2011 Higher Chemistry Past Paper Help

I'm always struggling with the multiple choice. Yeah, 25% chance I'll get it right but I have no idea how these answers are coming about. Can someone please explain them to me?

7. A pupil added 0·1 mol of zinc to a solution containing 0·05 mol of silver(I) nitrate.
Zn(s) + 2AgNO3(aq) → Zn(NO3)2(aq) + 2Ag(s) Which of the following statements about the experiment is correct?
A 0·05 mol of zinc reacts.
B 0·05 mol of silver is displaced.
C Silver nitrate is in excess.
D All of the zinc reacts.

10. Which of the following equations represents an enthalpy of combustion?
A C2H6(g) + 3 1/2 O2(g) ↓
2CO2(g) + 3H2O(l)
B C2H5OH(l) + O2(g) ↓
CH3COOH(l) + H2O(l)
C CH3CHO(l) + 1/2 O2(g) ↓
CH3COOH(l)
D CH4(g) + 1 1/2 O2(g) ↓
CO(g) + 2H2O(l)


14. A mixture of potassium chloride and potassium carbonate is known to contain 0·1 mol of chloride ions and 0·1 mol of carbonate ions.
How many moles of potassium ions are present?
A 0·15
B 0·20
C 0·25
D 0·30


15. Which of the following has the largest volume under the same conditions of temperature and pressure?
A 1g hydrogen
B 14 g nitrogen
C 20·2g neon
D 35·5 g chlorine

(Why neon? Because it's monatomic?)

16. 20 cm3 of butane is burned in 150 cm3 of oxygen.
C4H10(g) + 6 21 O2(g) → 4CO2(g) + 5H2O(g) What is the total volume of gas present after
complete combustion of the butane?
A 80 cm3
B 100 cm3
C 180 cm3
D 200 cm3

Any help would be appreciated!

7)
1 mole > 2 mole
0.05 Silver nitrate therefore 0.025 mole Zinc.
That instantly rules out A and D, as only 0.025 moles out of the total 0.1 mole of Zinc reacts. Also rules out C, as of course the Zinc is in excess as only 1/4 of it reacted, leaving you with B. That's how I'd do it anyway.

10)
The products of combustion are Carbon Dioxide and Hydrogen, so A. But we can rule out the rest of the answers as in B and C Carboxylic acids are formed, and those certainly aren't products of combustion. And Carbon monoxide would only be a product if there was a poor supply of oxygen, so not D.

14)
This is a tricky one.
You have
K(+) Cl(-) and (K+)2 CO3(2-), Therefore 0.2+0.1 = 0.3
0.1 0.1 0.2 0.1
You just gotta be able to write the equations out mate.

15)
Hydrogen, nitrogen and Chlorine are all diatomic, and in this case the mass given is half the GFM. However neon's GFM is 20.2g, which we have, so we have 1 mole of it = Largest volume.

16)
C4H10(g) + 6 21 O2(g) → 4CO2(g) + 5H2O(g)
1 vol 6.5 vol 4 vol 5 vol
20cm3 so ( 6.5 x 20 ) 4 x 20 5 x 20

20 130 80 100

We're told there are 150cm3 of oxygen, so ( 150 -130 ) 20cm3 in excess.

80 + 100 + 20 = 200cm3.

Hope this helped