Exam Questions

Question 1

Explain why the second ionisation energy for Mg is greater than the first (1)

Answer 1

As the electrons are being removed from a positive species, requiring more energy (1)

X ——> X+ + e- (state symbols missing)
X+—-> X2+ + e-

Question 2

A catalytic converter decreases the emissions of gases, such as carbon monoxide and nitrogen monoxide, from an internal combustion engine.
Describe the stages in a catalytic converter that result in this decrease. No equations are required.

Answer 2

• adsorption of CO and/or NO molecules on the catalytic surface (1)
• weakening of bonds
  (and chemical reaction between CO and NO) (1)
• desorption of CO2 and/or N2 /product (molecules) from the
  catalytic surface

Question 3

This question is about acids and bases.
A solution of methanoic acid, HCOOH, has a concentration of 0.240 mol dm−3 and a pH of
2.20.
Calculate the value of pKa for methanoic acid.

Answer 3

H+ = 10^-2.2 = 6.31 x 10-3
ka = (6.31 x 10-3)^2 / 0.24 = 1.66 x 10-4
pka = -log(1.66 x 10-4)
pKa = 3.7802

Question 4

Calculate the pH of the solution formed when
51.2 cm3 of 0.927 mol dm-3 NaOH(aq) is mixed with
40.4 cm3 of 0.370 mol dm-3 H2SO4(aq).
[Ionic product of water Kw = 1.00 × 10-14 mol2 dm-6]

Answer 4

2NaOH + H2SO4 ——> (Na)2SO4 + H20

mol of NaOH = 0.0474624
mol of H2SO4 = 0.014948

0.014948 x 2 = 0.029896

0.0474624 - 0.029896 = 0.0175664 mol of OH-

51.2 + 40.4 = 91.6 cm3
91.6/1000 = 0.0916

0.0175664/0.0916 = 0.191790393 conc of OH- in solution

1.00 × 10-14 / 0.191790393 = 5.214 x 10-14 - [H+]

pH = -log(5.214 x 10-14)
pH = 13.3

Question 5

This question is about the chemistry of hydrated magnesium nitrate, Mg(NO3)2.xH2O.
In an experiment, a sample of hydrated magnesium nitrate, Mg(NO3)2.xH2O, with a mass of
0.765 g, was dissolved in water and reacted with an excess of sodium hydroxide solution,
NaOH(aq).
The precipitate of magnesium hydroxide, Mg(OH)2 , produced was removed and dried. The
mass of the dried sample was 0.174 g.
Use the experimental data to calculate the value for x in the formula Mg(NO3)2.xH2O .
You must show all your working.

Answer 5

Mg(NO3)2.XH20 + 2NaOH —–> Mg(OH)2 + 2NaNO3

moles of Mg(OH)2 = 0.174/58.33 = 0.00298 mol (also = mol of Mg(NO3)2.xH2O due to 1:1)

molar mass of Mg(NO3)2.xH20 = 0.765/0.00298 = 256.8

molar mass of xH20 = 256.8 - 148.3 = 108.47

find x = 108.47/18 = 6.026

x = 6

Question 6

Explain why phosphorus forms PCl5 but nitrogen does not form NCl5.

Answer 6

• phosphorous can expand its octet
• whilst nitrogen can only accommodate 8 electrons