Chapter 13, Problem 33A

Interpretation Introduction

Interpretation: The statement that gases can easily become saturated with water vapor needs to be explained.

Concept Introduction: - Gases have small densities, it can be difficult to measure their mass for gases that are not soluble in water, it is possible to collect the evolved gas by displacement of water from a container.

Answer to Problem 33A

This is explained by collecting the gas by downward displacement of water.

Explanation of Solution

A gas is collected by the downward displacement of water contain some water vapors also exert some pressure called Aqueous pressure or tension.

Thus, aqueous tension is the pressure exerted by water vapor at a given temperature and it remains constant at a particular temperature.

Dalton law of partial pressure is applied.

  $\begin{array}{l}{\mathrm{P}}_{\mathrm{o}\mathrm{b}\mathrm{s}\mathrm{e}\mathrm{r}\mathrm{v}\mathrm{e}\mathrm{d}}=\mathrm{}{\mathrm{P}}_{\mathrm{d}\mathrm{r}\mathrm{y}\mathrm{}\mathrm{g}\mathrm{a}\mathrm{s}}+{\mathrm{P}}_{{\mathrm{H}}_2\mathrm{O}}(\mathrm{a}\mathrm{q}\mathrm{u}\mathrm{e}\mathrm{o}\mathrm{u}\mathrm{s}\mathrm{}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n})\\ {\mathrm{P}}_{\mathrm{d}\mathrm{r}\mathrm{y}\mathrm{}\mathrm{g}\mathrm{a}\mathrm{s}}=\mathrm{}{\mathrm{P}}_{\mathrm{o}\mathrm{b}\mathrm{s}}-\mathrm{}\mathrm{a}\mathrm{q}\mathrm{u}\mathrm{e}\mathrm{o}\mathrm{u}\mathrm{s}\mathrm{}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\end{array}$

This equation gives the pressure of the gas collected. Knowing the pressure of the collected gas, the number of moles ca also be calculated using ideal gas equation

  $\mathrm{P}\mathrm{V}=\mathrm{n}\mathrm{R}\mathrm{T}$

Conclusion

By downward displacement of water gases are collected in water.