Exam-style Questions: Atoms, Molecules and Stoichiometry

1.

Calculate the mean relative atomic mass of nickel from the mass spectrum.

(Marks available: 2)

Answer

Answer outline and marking scheme for question: 1

Intensities of peaks:



58Ni = 10.2 = 69.4% of total (abundance)



59Ni = 3.9 = 26.5% of total (abundance)



62Ni = 0.6 = 4.1% of total (abundance)



(Sum of intensities = 14.7)

Mean relative atomic mass = (58 x 69.4) + (59 x 26.5) + (62 x 4.1) =
58.4

100

(Marks available: 2)


2. In 1774 Joseph Priestly conducted one of his most famous experiments which
lead to a method for the preparation of oxygen. The experiment involved heating
a sample of mercury II oxide with a large lens.

The equation for this reaction is shown below:

2HgO(s) = 2Hg(l) + O2(g)

a) What volume of O2(g) would be obtained if 1.08g of mercury
II oxide were completely decomposed? (Given that 1 mole of a gas occupies 24
dm3 under the experimental conditions)



(Marks available: 3)

Answer

Answer outline and marking scheme for question: 2

Molar mass of HgO = 201 + 16 = 217 gmol-1



1.08g of HgO will contain 1.08 / 217 mols = 0.005mol

From the equation, 1 mole of O2(g) is produced from 2 moles of HgO



This means that 0.005 mol of HgO will produce 0.005 / 2 mol of O2 = 0.0025 mol




0.0025 mol of O2 will occupy 0.0025 x 24dm3 = 0.06dm3

(Marks available: 3)

3. An organic compound is found to have the following composition by mass:

C 72.7%; H 6.1%; N 21.2%.



a) Calculate the empirical formula of the compound.

(2 marks)

b) The molar mass of the compound is 198 gmol-1. Calculate
the molecular formula of the compound.

(3 marks)

(Marks available: 5)

Answer

Answer outline and marking scheme for question: 3

Ratio of elements in compound C : H : N



Ratio C : N : H = 72.7 / 12 : 6.1 / 1 : 21.2 / 14 = 6.1 : 6.1 : 1.5



Whole number ratio = 4 : 4 : 1



Empirical formula of compound is C4H4N

(Marks available: 5)