Question 15.6: Calculate the pH of an aqueous ammonia solution that has an ......

Chemistry [1089329]

Calculate the pH of an aqueous ammonia solution that has an $OH^{-}$ concentration of $1.9\times 10^{-3} M$ .

STRATEGY

First, calculate the $H_{3}O^{+}$ concentration from the $OH^{-}$ concentration, and then take the negative logarithm of $[H_{3}O^{+}]$ to convert to pH.

IDENTIFY
Known	Unknown
$[OH^{-}]=1.9\times 10^{-3}$	pH
$K_{w}=1.0\times 10^{-14}$

Step-by-Step

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[H_{3}O^{+}]=\frac{K_{w}}{[OH^{-}]}=\frac{1.0\times 10^{-14}}{1.9\times 10^{-3}}=5.3\times 10^{-12}  M

pH=-log  [H_{3}O^{+}]=-log  (5.3\times 10^{-12})=11.28

The pH is quoted to two significant figures (.28) because $[H_{3}O^{+}]$ is known to two significant figures (5.3).

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