The Gaseous State: Ideal & Real Gases & pV = nRT (Cambridge (CIE) A Level Chemistry): Exam Questions

State two basic assumptions of the kinetic theory as applied to an ideal gas.

Carbon dioxide does not behave as an ideal gas. 

Suggest one reason why not. 

Explain why CO₂ is a gas at room temperature.

Carbon dioxide can be used to inflate life jackets.

State and explain the effect of decreasing the temperature of the carbon dioxide on the pressure inside a life jacket. Assume the volume remains constant.

The kinetic theory of gases is used to explain the large scale (macroscopic) properties of gases by considering how individual molecules behave.

State two basic assumptions of the kinetic theory as applied to an ideal gas.

State two conditions under which the behaviour of a real gas approaches that of an ideal gas.

Deduce the order of decreasing ideal gas behaviour for the following gases.

ammonia, neon, nitrogen

most ideal .............................. > .............................. > .............................. least ideal

Explain your answer.

Explain, in terms of intermolecular forces and kinetic energy, why a liquid changes into a gas when its temperature is increased.

When magnesium reacts with hydrochloric acid, the following reaction occurs:

Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g)

During the reaction, the hydrogen produced occupies 103 cm³ at 25.0 °C and 100 kPa.

Calculate the amount, in mol, of hydrogen gas produced in this reaction. Show your working.

.................................... mol

Use your answer to (a) to calculate the volume, in cm³, of 0.150 mol dm^-3 HCl required to produce this amount of hydrogen gas. Give your answer to 3 significant figures. Show your working.

volume = ..................... cm³

In a separate experiment, 3.75 g of magnesium reacts with an excess of hydrochloric acid.

Calculate the mass, in g, of magnesium chloride produced. Show your working.

mass magnesium chloride = ....................... g

Phosphine, PH₃, is a gas formed by heating phosphorous acid, H₃PO₃, in the absence of air.

4H₃PO₃ (s) → PH₃ (g) + 3H₃PO₄ (s)

State the shape and bond angle in PH₃ (g).

3.45 x 10^-2 mol of H₃PO₃ is completely decomposed at a pressure of 100 kPa and 210 °C.

Calculate the volume occupied, in cm³, by the phosphine gas produced.

Show your working.

1.85 g of white phosphorus was reacted with 75.00 cm³ of 1.25 mol dm^-3 sodium hydroxide solution to make phosphine.

P₄ (s) + 3OH^- (aq) + 3H₂O (l) → PH₃ (g) + 3H₂PO₂^- (aq)

Show by calculation which reagent is in excess.

Show your working.

Use the information in part (c) to calculate the volume, in cm³, of phosphine that was produced at room conditions. Give your answer to 3 significant figures.

Oxygen exists as a diatomic gas, O₂ (g). A sample of O₂ (g) was made during a chemical reaction. When measured at 303 kPa and 28.0 °C the sample occupied a volume of 95.0 cm³.

Assume that oxygen behaves as an ideal gas under these conditions.

Calculate the mass, in g, of oxygen formed.

Show your working.

Use your answer to (a) to calculate the number of electrons involved in the bonding of this sample of oxygen.

Show your working.

O₂ (g) does not behave as an ideal gas. Explain why O₂ (g) deviates more from ideal gas behaviour at:

• very high pressures

• very low temperatures

The homologous series of alkanes undergo combustion with oxygen. A 2.00 dm³ flask contains 10.84 g of a gaseous alkane, X. The pressure in the flask is 300 kPa and the temperature is 20.0 °C.

Calculate the relative molecular mass, M_r, of X and hence deduce its molecular formula. Construct an equation for the complete combustion of X.

Show your working.

Airbags are safety devices fitted to modern cars. They are designed to rapidly inflate in the event of a collision, in order to protect the occupants of the car from the effects of the impact, and then quickly deflate.

The inflation of an airbag depends on the chemical decomposition of sodium azide, NaN₃.

The azide ion, N₃^–, contains one triple bond. Draw a ‘dot-and-cross’ diagram to show the arrangement of outer electrons present in an azide ion.

Suggest three properties of sodium azide. Explain your answer.

Sodium azide can be manufactured by reacting sodium amide, NaNH₂, with dinitrogen monoxide. Sodium hydroxide and ammonia are also produced.

550 g of sodium azide was produced in a reaction with a yield of 95.0%.

Construct an equation for this reaction. Hence calculate the mass, in g, of sodium amide required.

Show your working.

In a serious collision, the airbag deploys and rapidly fills with nitrogen as sodium azide decomposes.

2NaN₃ (s) → 2Na (s) + 3N₂ (g)

Assume nitrogen behaves as an ideal gas.

Calculate the mass of sodium azide that must decompose in order to inflate an airbag to a volume of 7.50 x 10^-2 m³ at a pressure of 150 kPa and a temperature of 35.0 °C.

Show your working.

Sodium azide is toxic. It can be destroyed by reacting it with acidified nitrous acid, HNO₂.

i) Complete the equation for this reaction:

2NaN₃ + ...HNO₂ + ...HCl → ...N₂ + ...NO + ...NaCl + 2H₂O

[2]

ii) Calculate the total volume of gas released, in dm³, when 75.0 g of sodium azide reacts with an excess of acidified nitrous acid and hydrochloric acid at room conditions.

Show your working.

[3]