Question

In the Dumas experiment, you measure the following data:
massgas + flask= 143.4316g
masscold water + flask= 385.75g
density of water= 0.9987g/mL
mass empty flask = 142.56g
Temperature of boiling water when the system is sealed = 105.3°C

A) Calculate the molar mass of the gas.
Te experiment, we weighed the "empty flask" when it was full of air. This is usually a minor error in an experiment, because we don't remove the air after an experiment, so the effect in other experiments is negligible. However, since we removed all of the air in the flask and replaced it with our gaseous unknown, it has a large effect on the molar mass calculations. Correct for the air in the "empty flask" measurement as follows:
B) Use the density of air at room temperature (1.225g/L) to determine the mass of air contained in the flask.
C) Subtract the mass of air from your original "empty flask" mass.
D) Compute the molar mass of the gas using the real mass of the empty flask.

Answer 1

Answer on Question #71724 - Chemistry - General Chemistry

Question:

In the Dumas experiment, you measure the following data:

massgas + flask = 143.4316g

masscold water + flask = 385.75g

density of water = 0.9987 g/mL

mass empty flask = 142.56g

Temperature of boiling water when the system is sealed = 105.3°C

A) Calculate the molar mass of the gas.

To experiment, we weighed the "empty flask" when it was full of air. This is usually a minor error in an experiment, because we don't remove the air after an experiment, so the effect in other experiments is negligible. However, since we removed all of the air in the flask and replaced it with our gaseous unknown, it has a large effect on the molar mass calculations. Correct for the air in the "empty flask" measurement as follows:

B) Use the density of air at room temperature (1.225 g/L) to determine the mass of air contained in the flask.

C) Subtract the mass of air from your original "empty flask" mass.

D) Compute the molar mass of the gas using the real mass of the empty flask.

Solution:

To determine the exact mass of air you need to know the volume of the flask. Based on the mass of the flask, I can assume that the experiment used a flask with a volume of 250 ml (cm³). Then, proceeding from this, the mass of air in the flask will be empty:

$$m = V \cdot d = 0.25 \cdot 1.225 = 0.30625 \text{ g};$$

Real mass empty flask = 142.56 - 0.30625 = 142.2538 g;

Mass gas: (mass gas + flask) - (real mass empty flask) = 143.4316 - 142.2538 = 1.17785 g;

m (H₂O) = (mass cold water + flask) - (mass gas + flask) = 385.75 - 143.4316 = 242.3184 g = 0.242 kg.

$$\Delta T = \varepsilon \cdot m = \varepsilon \cdot \frac{m(gas)}{M(gas) \cdot m(H_2O)};$$ $$\varepsilon(H_2O) = 0.52;$$ $$M(gas) = \frac{\varepsilon \cdot m(gas)}{\Delta T \cdot m(H_2O)} = \frac{0.52 \cdot 1.17785}{5.3 \cdot 0.242} = 0.5 \text{ g/mol}$$

Answer: 0.5 g/mol.

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