*Dalton’s Law:*

• at 25°C, water has a vapor pressure of 23.78 mmHg

$\overline{){{\mathbf{P}}}_{{\mathbf{total}}}{\mathbf{=}}{{\mathbf{P}}}_{\mathbf{Gas}\mathbf{}\mathbf{1}}{\mathbf{+}}{{\mathbf{P}}}_{\mathbf{Gas}\mathbf{}\mathbf{2}}{\mathbf{+}}{{\mathbf{P}}}_{\mathbf{Gas}\mathbf{}\mathbf{3}}{\mathbf{.}}{\mathbf{.}}{\mathbf{.}}}\phantom{\rule{0ex}{0ex}}{\mathbf{P}}_{\mathbf{total}}\mathbf{=}{\mathbf{P}}_{{\mathbf{H}}_{\mathbf{2}}}\mathbf{+}{\mathbf{P}}_{\mathbf{water}}\phantom{\rule{0ex}{0ex}}\mathbf{791}\mathbf{}\mathbf{mm}\mathbf{}\mathbf{Hg}\mathbf{=}{\mathbf{P}}_{{\mathbf{H}}_{\mathbf{2}}}\mathbf{+}\mathbf{23}\mathbf{.}\mathbf{78}\mathbf{}\mathbf{mm}\mathbf{}\mathbf{Hg}$

${\mathit{P}}_{{\mathbf{H}}_{\mathbf{2}}}\mathbf{=}$**767.22 mmHg**

**Calculate ****no. of moles of H _{2} gas:**

$\frac{\mathbf{PV}}{\mathbf{RT}}\mathbf{=}\frac{\mathbf{n}\overline{)\mathbf{RT}}}{\overline{)\mathbf{RT}}}\phantom{\rule{0ex}{0ex}}\overline{){\mathbf{n}}{\mathbf{=}}\frac{\mathbf{PV}}{\mathbf{RT}}}\phantom{\rule{0ex}{0ex}}\mathbf{n}\mathbf{=}\frac{\left(\mathbf{767}\mathbf{.}\mathbf{22}\mathbf{}\overline{)\mathbf{mm}\mathbf{}\mathbf{Hg}}{\displaystyle \frac{\mathbf{}\mathbf{1}\mathbf{}\overline{)\mathbf{atm}}}{\mathbf{}\mathbf{760}\mathbf{}\overline{)\mathbf{mm}\mathbf{}\mathbf{Hg}}}}\right)\left(\mathbf{0}\mathbf{.}\mathbf{899}\mathbf{}\overline{)\mathbf{L}}\right)}{\left(\mathbf{0}\mathbf{.}\mathbf{08206}\frac{\overline{)\mathbf{L}}\mathbf{\xb7}\overline{)\mathbf{atm}}}{\mathbf{mol}\mathbf{\xb7}\overline{)\mathbf{K}}}\right)\left(\mathbf{25}\mathbf{\xb0}\mathbf{C}\mathbf{+}\mathbf{273}\mathbf{.}\mathbf{15}\right)\mathbf{}\overline{)\mathbf{K}}}$

*n = 0.0371 mol H _{2}*

Balanced equation: Zn(s) + 2HCl(aq) → H_{2}(g) + ZnCl_{2}(aq)

- Mole to mole comparison: 1 mole Zn forms 1 mole H
_{2}

Modern pennies are composed of zinc coated with copper. A student determines the mass of a penny to be 2.483 g and then makes several scratches in the copper coating (to expose the underlying zinc). The student puts the scratched penny in hydrochloric acid, where the following reaction occurs between the zinc and the HCl (the copper remains undissolved):

Zn(s) + 2HCl(aq) → H_{2}(g) + ZnCl_{2}(aq)

The student collects the hydrogen produced over water at 25 ^{o}C. The collected gas occupies a volume of 0.899 L at a total pressure of 791 mm Hg .

Calculate the percent zinc in the penny. (Assume that all the Zn in the penny dissolves.)

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