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

This question is about ideal gas theory. 

State the 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. 

Explain the effect of decreasing the temperature of carbon dioxide on the pressure inside a life jacket.

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.

Place the following gases in decreasing order of ideal behaviour.

ammonia, neon, nitrogen

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

Explain your answer.

Explain why a liquid eventually becomes a gas as the 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 moles, of hydrogen gas produced during the reaction. 

.................................... moles

Using your answer to part (a), calculate the exact volume in cm³ of 0.15 mol dm^-3 HCl, which would be needed to produce that amount of hydrogen gas. Give your answer to 1 decimal place.

volume = ..................... cm³

A student uses the reaction in part (a) to complete a separate experiment using 3.75 g of magnesium. 

Calculate the mass, in grams, of magnesium chloride produced by the student during this reaction.

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 × 10⁻² mol of H₃PO₃ is completely decomposed at 100 kPa pressure and 210 °C.

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

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)

Deduce which chemical is the limiting reagent.  

Use the information in part (c) to calculate the volume, in cm³, of phosphine that was produced at room temperature and pressure.  Give your answer to three 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 °C the sample occupied a volume of 95.0 cm³.

Calculate the mass of oxygen formed.

For this calculation, assume that oxygen behaves as an ideal gas under these conditions.

Calculate the number of electrons involved in the bonding of this sample of oxygen. You may find it helpful to use your answer to (a).

O₂ (g) does not behave as an ideal gas under these conditions. 

 Explain why O₂ (g) behaves even less ideally at:

• very high pressures

• very low temperatures

The homologous series of alkanes undergo combustion with oxygen.

A 2.0 dm³ flask contains 10.84 g of a gaseous alkane, X. The pressure in the flask is 300 kPa and the temperature is 20 ^oC.

Write an equation for the complete combustion of X.

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.

550 g of sodium azide, sodium hydroxide and ammonia were produced during the chemical reaction of sodium amide, NaNH₂, and dinitrogen monoxide. The yield of sodium azide in this reaction was 95.0%.

Calculate the mass, in grams, of sodium amide required for this reaction.

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

2NaN₃ (s) → 2Na (s) + 3N₂ (g)

Calculate the mass of sodium azide that must decompose in order to inflate an airbag to a volume of 7.50 × 10⁻² m³ at a pressure of 150 kPa and temperature of 35 °C.

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

i) Balance the chemical equation for the reaction of sodium azide with acidified nitrous acid  

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

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 ii) Calculate the volume of gas released, in dm³, when 250 cm³ volume of nitrous acid is used to destroy 75.0 g of sodium azide at room temperature and pressure. 

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