In an experiment to determine the relative molecular mass of a volatile liquid a sample of the liquid is vaporised at a given temperature and pressure and its volume measured - Leaving Cert Chemistry - Question 3 - 2003

1. Apparatus Setup: Use a flask with a small pinhole covered to allow vapor escape. Label the flask.
2. Mass Measurement: Weigh the empty flask and then add the liquid. Weigh again to find the mass of the vaporised liquid by subtraction.
3. Volume Measurement: Fill a graduated measuring cylinder with water and invert the flask submerged in it. Measure the volume of water displaced to find the volume of vapor.
4. Temperature Measurement: Use a thermometer to read the temperature of the water, ensuring it remains constant during the experiment.

The pressure can be measured using a barometer or a bourdon gauge. A pressure sensor can also be used, positioned within the setup to provide accurate readings. Using a barometer, ensure that the liquid is at room temperature or near the boiling point for accurate measurement.

Given:

• Mass = 0.275 g
• Volume = 95 cm³ = 95 x 10⁻⁶ m³
• Pressure = 1 x 10⁵ Pa
• Temperature = 97 °C = 370 K (converted using T(K) = T(°C) + 273)
  Using the ideal gas law:
  $n = \frac{PV}{RT}$
  Substituting values:
  $n = \frac{(1 \times 10^5) \times (95 \times 10^{-6})}{(8.31) \times (370)}$
  Calculating gives:
  $n \approx 0.00301 \text{ moles}$

To find the relative molecular mass (M):
Using the formula:
$M_r = \frac{mass \ of \ substance}{number \ of \ moles}$
Substituting the values:
$M_r = \frac{0.275}{0.00301} \approx 91.0 \text{ g/mol}$
Therefore, the relative molecular mass of the volatile liquid is approximately 91.0 g/mol.