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Gas collection and measurement

Key idea: Collect gas by a method that avoids dissolving or losing it, and record volume, temperature and pressure. Subtract water-vapour pressure from a wet sample before using pV = nRT for the product gas.

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  • GCE A-Level H2 Chemistry 2027
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A gas volume can tell you how much reaction has occurred, but only if you know what gas was measured and the conditions of measurement. Begin with the collection method; apply pV = nRT afterwards.

Check your understanding 1 Choose the gas pressure

A sample collected over water contains only product gas and water vapour. Which pressure belongs in pV = nRT when finding the product amount?

Pressure to use
Show answer

Use the product partial pressure: subtract the water-vapour contribution from the total. Both gases share the collected volume.

Choose a method that keeps the gas

MethodWhen it can workWhat to check
Gas syringeGas can be transferred to the syringe without reacting with itConnections are gas-tight; the plunger moves freely; the capacity is sufficient
Collection over waterGas is sufficiently insoluble in water and does not react appreciably with itDelivery tube enters beneath the inverted vessel; the gas contains water vapour
Collection by displacement of airGas density and reactivity make the chosen orientation suitableMixing with air limits purity; this is not usually a precise quantitative volume method

A gas that dissolves substantially in water will be under-collected by that method. A syringe avoids that particular loss. Gas produced from an aqueous mixture may still contain water vapour, even when it is collected in a syringe.

Gas collection over water setupA delivery tube enters beneath the open end of an inverted measuring cylinder in a water trough. Bubbles rise into a gas space containing dry gas and water vapour. The water level inside the cylinder is higher than the level outside, so the total gas pressure must not simply be assumed to equal atmospheric pressure. The schematic is not to scale.Inverted measuring cylinderWater troughCollected gasdry gas +water vapourwater levelDelivery tube enters below the open end;gas bubbles rise and displace water.Pressure bookkeepingp(total) = p(dry gas) + p(H₂O)Use dry-gas pressure in pV = nRT.Subtract water vapour pressure first.
Gas displaces water in an inverted vessel. The gas space contains product gas and water vapour. The unequal water levels shown mean its pressure cannot simply be assumed equal to atmospheric pressure.

Notice where the delivery tube ends: beneath the open mouth of the inverted vessel. Gas bubbles enter the vessel instead of escaping elsewhere in the trough. Read the collected volume at the gas–water boundary using the vessel’s scale.

Scope of the H2 practical examination

The 9476 syllabus includes gas collection and measurement, but says students are not required to carry out collection over water or with a gas syringe in the practical examination. These methods still help you interpret an investigation and its data.

Keep temperature, pressure and composition separate

Record the temperature of the gas when measuring its volume. Let it reach the measurement temperature where the method requires this; a freshly produced warm gas has a different volume from the same amount after cooling.

For gas collected over water:

pₜₒₜₐₗ = p_(product gas) + p_H₂O

Subtract the water-vapour pressure at that temperature before calculating the amount of product gas, assuming no other gases are present. The total pressure can be taken as atmospheric pressure only when the water levels inside and outside the vessel are equal, or when an appropriate pressure correction is supplied. The diagram shows unequal levels.

Air initially trapped in the collection space also matters. A calculation treating the whole sample as pure product gas would then overestimate the amount of product present. In a displacement measurement, apparatus dead space and the starting reading must be accounted for according to the method.

For SI calculations use p in Pa, V in m³, T in K and R = 8.31 J mol⁻¹ K⁻¹. Remember: 1 cm³ = 10⁻⁶ m³.

Worked example 1

Calculate the amount of a gas collected over water

Problem

A 96.0 cm³ sample contains only product gas and water vapour at 298 K. Its measured total pressure is 100 kPa; water-vapour pressure is 3.00 kPa. Find the amount of product gas.
Show full solution

  1. Choose the pressure

    Method

    Subtract the water-vapour contribution.

    Reason

    The required amount is that of product gas, not the whole wet sample.

    Working

    Answer

    p_product = 100-3.00 = 97.0 kPa = 9.70 × 10⁴ Pa

  2. Use consistent units

    Method

    Convert the volume and solve n = pV/(RT).

    Reason

    The numerical value of R requires SI pressure and volume with temperature in kelvin.

    Working

    Answer

    V = 9.60 × 10⁻⁵ m³
    n = ((9.70 × 10⁴)(9.60 × 10⁻⁵))/8.31(298) = 3.76 × 10⁻³ mol

Check your understanding 2 Calculate the corrected amount

A wet sample contains only product gas and water vapour: 80.0 cm³ at 300 K and measured total pressure 101 kPa. Water-vapour pressure is 3.50 kPa. Use R = 8.31 J mol⁻¹ K⁻¹ to find product pressure in Pa, volume in m³ and product amount.

Show the steps Optional working
Unit: Pa
Unit: m³
Unit: mol
Show answer

p = 97.5 kPa = 97500 Pa; V = 8.00 × 10⁻⁵ m³. Hence n = 97500(8.00 × 10⁻⁵)/[8.31(300)] = 3.13 × 10⁻³ mol. Using total pressure gives 3.24 × 10⁻³ mol and counts water vapour too.

Obtain a reliable volume

Check the gas-tight connections before starting. Coordinate the start of reaction with the start of collection, so early gas is not lost while fitting a stopper. Use the actual starting syringe reading rather than assuming zero. A leak before the collector lowers the collected volume and, under otherwise unchanged conditions, the calculated amount.

Provide a free path for the gas

A blocked delivery tube or jammed syringe plunger can let pressure build up. Use the instructed apparatus and check its operation before the supervised experiment. Gas-specific hazards may require ventilation or a fume cupboard; never identify a gas by directly smelling it.

Check your understanding 3 Choose the correction and diagnose a loss

A dry gas sample of 60.0 cm³ is measured at 100 kPa and 300 K. Calculate its amount. Choose the pressure that fits this dry sample; explain that choice and a leak in the following question.

Show the steps Optional working
Unit: m³
Unit: mol
Show answer

Using SI units:

n = ((1.00 × 10⁵)(6.00 × 10⁻⁵))/8.31(300) = 2.41 × 10⁻³ mol

Check your understanding 4 Show your method, pressure choice and leak correction

Show the calculation for 60.0 cm³ of dry gas at 100 kPa and 300 K, using R = 8.31 J mol⁻¹ K⁻¹. Explain the pressure choice, the effect of a leak and a matching improvement.

Show answer

Using SI units:

n = ((1.00 × 10⁵)(6.00 × 10⁻⁵))/8.31(300) = 2.41 × 10⁻³ mol

No water-vapour correction is needed because the sample is explicitly dry. Escaping gas reduces the volume reaching the collector, so the calculated collected amount is too small. Secure and check that connection before repeating; averaging several leaky trials would not replace the lost gas.

A useful result reports the collection method, measured volume, temperature, pressure and any correction. “Gas was collected” leaves too much of the measurement model unstated.

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Syllabus and review details

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