Answer
a. $[\mathrm{H}^{+}]\approx 2.5\times 10^{-5}$ M
b. $[\mathrm{H}^{+}]\approx 5.0\times 10^{-8}$ M
Work Step by Step
(see p.381)
The $\mathrm{p}\mathrm{H}$ scale measures the acidity of a solution:$ \quad \mathrm{p}\mathrm{H}=-\log[\mathrm{H}^{+}]$
where $[\mathrm{H}^{+}]$ is the hydrogen ion concentration (in moles per liter, M).
Solutions are
neutral if $pH=7$
acidic if $pH < 7$
basic if $pH > 7$
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Solving for $[\mathrm{H}^{+}]$,
$pH=-\log[\mathrm{H}^{+}]\qquad /\times(-1)$
$\log[\mathrm{H}^{+}]=-pH\qquad /$... apply $10^{(...)}$ to both sides
$[\mathrm{H}^{+}]=10^{-pH}$
a. $pH=4.6\ \ \Rightarrow\ \ $
$[\mathrm{H}^{+}]=10^{-4.6} \approx 2.5\times 10^{-5}$ M
b. $pH=7.3\ \ \Rightarrow\ \ $
$[\mathrm{H}^{+}]=10^{-7.3}\approx 5.0\times 10^{-8}$ M
