Sodium azide,

CHEMISTRY · Multi-part · A Level · Chemistry · Amount of substance · Amount of substance

Problem

Sodium azide,

\text{NaN}_3

, is used to inflate automobile air bags by thermal decomposition according to the balanced equation

2\,\text{NaN}_3(s) \rightarrow 2\,\text{Na}(s)+3\,\text{N}_2(g)

A chemist decomposes

0.100\,\text{mol}

\text{NaN}_3

in a rigid

2.50\,\text{L}

steel reaction vessel that is open to a mercury manometer via a stopcock. The vessel is submerged in a

45^{\circ}\text{C}

water bath and connected to an inverted graduated cylinder filled with the same water so that the nitrogen gas produced displaces water into the bath. When gas collection is complete and the water levels inside and outside the cylinder are equal, the manometer indicates a total gas pressure of

1.20\,\text{atm}

. The vapor pressure of water at

45^{\circ}\text{C}

71.9\,\text{mmHg}

. Assume

R = 0.0821\,\text{L}\cdot\text{atm}\cdot\text{mol}^{-1}\cdot\text{K}^{-1}

. (a) Determine the amount, in moles, of

\text{N}_2

actually present in the vessel under the collection conditions. (b) Using stoichiometry, calculate the theoretical number of moles of

\text{N}_2

that should have been produced. Then compute the percent yield of nitrogen for this experiment. (c) After the stopcock is closed, the vessel is allowed to cool to

25^{\circ}\text{C}

while the volume remains constant. Calculate the new total pressure inside the vessel, assuming the amount of water vapor remains unchanged. (d) State one assumption about the saturation of water vapor that you made in part (c) and discuss how a deviation from that assumption would affect the calculated pressure.

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