Question

A solution of an acid has pH = 4.70. Find out the number of $OH^-$ ions $(pK_w = 14)$

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

Answer 1

Answer: (A)

$pH=4.70 \Rightarrow -log\left[H^{+}\right]=4.70$
$\therefore log \left[H^{+}\right]=-4.7=\bar{5}\cdot30$
or $\left[H^{+}\right]=1.999 \times10^{-5}=2\times10^{-5}$
$pK_{w}=14 \Rightarrow \left[H^{+}\right]\times\left[OH^{-}\right]=10^{-14}$
$\therefore \left[OH^{-}\right]=\frac{10^{-14}}{2\times10^{-5}}=0.5\times10^{-9}$
$=5\times10^{-10}$