Experimental work Question 1

Paper 3 Section A Experimental work Question 1

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A student devised an experiment to determine the molar mass of an unknown gas X.

Firstly, he filled a glass gas syringe (accurate to ± 0.5 cm3) with 100 cm3 of air then placed a rubber seal over the nozzle and weighed the syringe.

He then emptied the gas syringe, refilled it with 100 cm3 of the unknown gas X, replaced the rubber seal and reweighed the syringe.

Finally, he measured the temperature of the room.

He obtained the following data:

Mass of syringe + 100 cm3 of air186.293 ± 0.001 g
Mass of syringe + 100 cm3 of unknown gas X186.358 ± 0.001 g
Temperature20.0 ± 0.5 oC

In order to calculate the mass of the unknown gas X the student made the following assumptions:

The atmospheric pressure = 100 kPa

Air contains 80% nitrogen and 20% oxygen by volume so has a ‘molar mass’ equivalent to 28.8 g mol-1.

Due to Archimedes' Principle, a syringe containing 100 cm3 of air appears to have the same mass as a syringe containing 0 cm3 of air.

(a) Determine the mass of 100 cm3 of air at 20 oC. [2]

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(b) Determine the mass of 100 cm3 of X at 20 oC. [1]
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(c) Show that the molar mass of X is equal to 44.7 g mol−1 [1]

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(d) The accepted value for the molar mass of X is 44.0 g mol−1. Calculate the percentage error in the student’s result. [1]
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(e) Identify, with a reason, the piece of equipment used that had the largest percentage uncertainty associated with the result. [1]
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