3.1 - Determination of the Relative Molecular Mass of a Volatile Liquid

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Experiment Summary

This experiment uses the ideal gas equation $PV = nRT$ to estimate the relative molecular mass ( $M_r$ ) of a volatile liquid. The liquid is vaporised in a container at a known temperature, volume, and pressure, allowing the number of moles of vapour to be calculated.

By measuring the mass of the vapour, the relative molecular mass is determined.

Materials and Apparatus Required

Chemicals

Volatile liquid (e.g., propanone)
Water

Apparatus

250 cm³ conical flask
600 cm³ beaker
Aluminium foil
Clamp
Rubber band
Bunsen burner or hotplate
Tripod and gauze
Thermometer
Barometer
Electronic balance
100 cm³ graduated cylinder
Pin
Dropping pipette

Safety Precautions

Wear safety glasses at all times.
Propanone is highly flammable. Ensure that the flask is far from open flames when adding the liquid.
Use a well-ventilated area to avoid inhaling propanone vapours.
Handle hot water and glassware with care to avoid burns.

Method

Two-thirds fill a beaker with water and heat it to approximately 95°C using a Bunsen burner or hotplate.
Record the mass of the dry conical flask, aluminium foil cap, and rubber band.
Use a pipette to add 3–4 cm³ of propanone to the flask.
Secure the flask with the foil cap and rubber band, then poke a small hole in the foil with a pin.
Immerse the flask in the hot water bath, ensuring no vapour escapes except through the pinhole.
The liquid will vaporise, and once it appears fully vaporised, remove the flask from the water.
Measure the temperature of the water, which represents the temperature of the vapour.
Let the flask cool, then measure its mass again. The difference in mass gives the mass of the vapour.
Fill the flask with water, then transfer the water to a graduated cylinder to find the flask's volume.
Use a barometer to record the atmospheric pressure.
Calculate the relative molecular mass ( $M_r$ ) of the volatile liquid using the ideal gas equation.

Results

Measurement	Value
Mass of flask, cap, and rubber band	g
Mass of flask, cap, rubber band, and vapor	g
Mass of vapor	g
Atmospheric pressure	mmHg/Pa
Temperature of boiling water	°C/K
Volume of flask	cm³/m³

Sample Calculation

Ideal Gas Equation:

PV = nRT

Calculate the number of moles of vapour (n):

n = \frac{PV}{RT}

Relative Molecular Mass:

Mr = \frac{m}{n}

Example Questions with Answers

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Q1: Why is it necessary for the liquid to be volatile?

The liquid must easily vaporise at the temperature used in the experiment so that the ideal gas law can be applied accurately.

This ensures the vapour behaves similarly to an ideal gas.

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Q2: What other method could be used to determine the relative molecular mass?

Mass spectrometry can provide highly accurate measurements of molecular masses.

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Q3: Which measurement is most likely to introduce inaccuracy and why?

Volume measurement is prone to error because real gases deviate from ideal behaviour, particularly under conditions of high pressure or low temperature.

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Q4: How would a small drop of water in the flask affect the results?

The drop of water would vaporise and increase the volume of the gas, leading to an inaccurately high volume reading and a smaller calculated $M_r$ .

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Q5: How would you calculate the density of the vapour at the boiling water temperature?

Use the formula:

\text{Density} = \frac{\text{mass}}{\text{volume}}

For example, using sample data from the experiment:

\text{Density} = \frac{0.52 \, \text{g}}{284 \, \text{cm}³} = 0.0018 \, \text{g/cm}³

- Determination of the Relative Molecular Mass of a Volatile Liquid Simplified Revision Notes for Leaving Cert Chemistry

3.1 - Determination of the Relative Molecular Mass of a Volatile Liquid

Experiment Summary

Materials and Apparatus Required

Chemicals

Apparatus

Safety Precautions

Method

Results

Sample Calculation

Example Questions with Answers

Q1: Why is it necessary for the liquid to be volatile?

Q2: What other method could be used to determine the relative molecular mass?

Q3: Which measurement is most likely to introduce inaccuracy and why?

Q4: How would a small drop of water in the flask affect the results?

Q5: How would you calculate the density of the vapour at the boiling water temperature?

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