Question

A solution of an acid has pH = 4.70. Find out the number of $O{H}^{-}$ ions $(p{K}_w=14)$

• A. $5\times 10^{-10}$
• B. $4\times 10^{-10}$
• C. $2\times 10^{-5}$
• D. $9\times 10^{-4}$

Answer 1

Answer: (A)

$pH=4.70\Rightarrow -log\left[H^{+}\right]=4.70$
$\therefore log\left[H^{+}\right]=-4.7=\bar{5}\cdot 30$
or $\left[H^{+}\right]=1.999\times 10^{-5}=2\times 10^{-5}$
$p{K}_w=14\Rightarrow \left[H^{+}\right]\times \left[O{H}^{-}\right]=10^{-14}$
$\therefore \left[O{H}^{-}\right]=\frac{10^{-14}}{2\times 10^{-5}}=0.5\times 10^{-9}$
$=5\times 10^{-10}$