2007 NJC Prelims Paper 2 FAQs

Explanation for 2c
Total pressure = Partial pressure of O2 + Partial pressure of H2O
Partial pressure of O2 = 101 kPa - 42.3 kPa = 58.7 kPa
Let n be no. of mol of O2 collected in jar.
(58.7 x 10^3)(87 x 10^-6) = n(8.31)(30+273)
n = 2.03 x 10^-3

Explanation for 3b(ii)
In 3b(i), you found amount of heat required to raise the temperature of the calorimeter by 1 degree Celsius.
In 3b(ii), temperature rise = 2 degree Celsius. Find amount of heat evolved. That is for 0.5 g of aspirin burnt. How much heat is evolved if 1 mol of aspirin is burnt?

Explanation for 5c(ii)
0.12 mol of Cl2 take part in both reactions (1) and (2). Let x mol of Cl2 react in (1) and 0.12-x mol of Cl2 react in (2). Solve for x and hence evaluate 0.12-x.