Exam-style Questions: States of Matter

1. Hydrogen generated from the reaction between magnesium and nitric acid was
collected. At a temperature of 30°C and pressure

of 1.2 x 105 Nm-2, 340cm3 of gas
was collected.

What would the volume of this gas be at s.t.p.?

(Marks available: 2)

Answer

Answer outline and marking scheme for question: 1

Reaction conditions: P1 = 1.2 x 105 Nm-2 ; T1 = 303K ; V1 = 340cm3


S.t.p.


P2 = 1.01 x105 Nm-2 ; T2 = 273K (1)

P1V1 = P2V2 (1.2
x 105)(340)
= (1.01 x105)V2

T1 T2 303
273

V2 = 364.0 cm3

(Marks available: 2)

2. The ideal gas equation can be written as 'PV = nRT'



a) Use this equation to calculate the volume of 0.5 moles of an ideal
gas at 350K and 1x 105Nm-2 pressure.

(2 marks)

b) A compound was vaporised and it was found that 0.15g of the gas occupied
120cm3 at 323K and 100 kPa.



Calculate the molecular mass of the compound.

(2 marks)

(Marks available: 4)

Answer

Answer outline and marking scheme for question: 2

a) Rearranging the equation: V = nRT/P

V = 0.5 (8.314)(350) / (1 x 105)



V = 0.015 m3 (numerical = 1, units = 1)

(2 marks)

b) Using PV = nRT and n = m / Mr



n = PV / RT = (1 x 105)( 0.00012) / (8.314)(323)



n = 0.004



Mr = 0.15 / 0.004 = 37.5

(2 marks)

(Marks available: 4)

3. Explain in terms of structure and bonding:

a) Why graphite conducts electricity but diamond does not.

(3 marks)



b) Why water is a liquid at room temperature but hydrogen sulphide is
a gas.

(2 marks)

(Marks available: 5)

Answer

Answer outline and marking scheme for question: 3

a) The carbon atoms in graphite are covalently bonded to three other
carbon atoms and have delocalised electrons in between the sheets of atoms.

(1 mark)

These delocalised electrons are able to move when a p.d. is applied

(1 mark)

The carbons in diamond are covalently bonded to four other carbon
atoms so there are no electrons able to move freely.

(1 mark)

(max 3 marks)

b) Water (H2O) has a higher boiling point due to there being
a lot of hydrogen bonding between the molecules.

Hydrogen sulphide (H2S) has a much lower boiling point because there
is a much lower degree of hydrogen bonding.

(2 marks)

(Marks available: 5)