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Example Question - 2

A student determined the relative molecular mass, M_r, of an unknown volatile liquid Y in an experiment as shown in the diagram.

The student used a hypodermic syringe to inject a sample of liquid Y into a gas syringe in an oven.At the temperature of the oven, liquid Y vaporised.

Mass of hypodermic syringe and liquid Y before injection 10.91 g

Mass of hypodermic syringe and liquid Y after injection 10.70 g

Oven temperature 98.1 °C

Atmospheric pressure 102 kPa

Increase in volume in gas syringe after injection of Y 85.0 cm3

Use the results to determine the relative molecular mass of Y.The gas constant R = 8.31 J K–1 mol–1
solution mass of Y = 0.21gM_r = mass × (RT÷pV)M_r = (0.21×8.31×371.1)÷(102000×85×10^-6) M_r=74.7

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Specification: AQA Chemistry (7404), AS-level, 3.1.2.1 Relative atomic mass and relative molecular mass. Topic: Amount of Substance.

Key terms in this lesson

Relative molecular mass: The weighted mean mass of a molecule compared to 1/12th the mass of a carbon-12 atom.

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