Paper 3

Question 1

Give a chemical explanation for why water has a concentration of approx 56moldm-3

Answer 1

1dm3 water has a mass 1000g.
n(H2O) in 1dm3 = 1000/18 = 56mol

Question 2

Give a chemical explanation for why the reaction of ethane with chlorine under UV radiation is a poor method for preparing a high yield of chloroethane

Answer 2

Further substitution occurs

Question 3

The value of Kw at 37degrees is 2.4x10^-14.
Calculate the pH of water.

Answer 3

Kw = [H+][OH-].
[H+] = root(2.4x10^-14) = 1.55 × 10–7 
pH = –log (1.55 × 10–7) = 6.81

Question 4

Why would use of excess HCOOH reactant ensure that the order with respect to HCOOH is effectively zero?

Answer 4

Concentration of HCOOH would be constant