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To determine the relative molecular mass (M_r) of compound X, a volatile liquid, a small volume of the pure liquid was vaporised in a suitable container - Leaving Cert Chemistry - Question 3 - 2019
Question 3
To determine the relative molecular mass (M_r) of compound X, a volatile liquid, a small volume of the pure liquid was vaporised in a suitable container. The atmosph... show full transcript
Worked Solution & Example Answer:To determine the relative molecular mass (M_r) of compound X, a volatile liquid, a small volume of the pure liquid was vaporised in a suitable container - Leaving Cert Chemistry - Question 3 - 2019
Step 1
Draw a large, labelled diagram of an arrangement of apparatus you used to carry out this experiment.
Answer
To answer this part, draw a conical flask that is approximately half immersed in a beaker of water, with a lid made of aluminum foil or a bung. Include a pinhole to allow gas to escape and show the vapor line above the liquid. Use a thermometer to measure the temperature of the vapor, positioned near the top. Ensure the thermometer tube is inserted correctly to get accurate temperature readings.
Step 2
With reference to your diagram, explain why the pressure of the vapour at the end of the heating stage of the experiment was known to be equal to the atmospheric pressure at your location.
Answer
At the end of the heating stage, the vapor in the flask is at its boiling point and in equilibrium with its liquid form. Since the vapor can escape through the pinhole, the pressure exerted by the vapor equals the atmospheric pressure surrounding the flask. This is supported by the presence of an open space in the apparatus allowing vapor interplay with the atmosphere.
Step 3
With reference to your diagram, explain how the volume of the vapour was measured.
Answer
The volume of vapor was measured using a gas syringe or graduated cylinder. Initially, the syringe was filled with a known volume of air or another gas. After vaporization, the amount of vapor displaced the air in the syringe. By measuring the final volume in the syringe, subtracting the initial volume provides the volume of the vapor contained.
Step 4
With reference to your diagram, describe how the mass of the vapour in the container at the end of the heating stage was found.
Answer
To find the mass of the vapor, the mass of the flask plus the liquid and the foil lid was measured first. After the heating stage, the remaining liquid and vapor contents were weighed. The mass of the vapor is determined by subtracting the initial mass of the flask from the mass measured after the experiment, allowing for calculation of only the vapor's contribution.
Step 5
Calculate, correct to the nearest whole number, the relative molecular mass of X.
Answer
To calculate the relative molecular mass, use the equation:
Given the parameters:
- Pressure (P) = 1.011 × 10⁵ Pa
- Volume (V) = 76 cm³ = 76 × 10^{-6} m³
- R = 8.31 J/(mol·K)
- Temperature (T) = 99 °C = 372.15 K
Substituting values:
Then, the relative molecular mass, M_r, can be calculated as:
.
Step 6
Account for the volatility of bromine compared to that of water.
Answer
Bromine (Br₂) has a significantly higher relative molecular mass than water (H₂O), resulting in stronger intermolecular forces. Water exhibits hydrogen bonding, which creates high cohesion and boiling point due to higher energy needed to break these interactions. Bromine molecules are only subjected to weaker van der Waals forces due to their non-polar nature, allowing them to vaporize more readily. Thus, bromine's lower boiling point (58.8 °C) compared to water (100 °C) can be attributed to these factors.