Question

The number of hydroxyl ions in $100 \,mL$ of a solution having $pH \,10$ is

• A. $1 \times 10^{4}$
• B. $3.012 \times 10^{4}$
• C. $6.02 \times 10^{18}$
• D. $6.023 \times 10^{19}$

Answer 1

Answer: (C)

Given, $pH =10$
$\therefore \left[ H ^{+}\right]=1 \times 10^{- pH }=1 \times 10^{-10}\, mol / L$
$\therefore \left[ OH ^{-}\right]=\frac{1 \times 10^{-14}}{1 \times 10^{-10}}=1 \times 10^{-4}\, mol / L$
$\because$ The number of hydroxyl ions present in $1 \, mol =6.023 \times 10^{23}$
$\therefore $ The number of hydroxyl ions in
$1 \times 10^{-4} \, mol / L =6.023 \times 10^{23} \times 10^{-4} / L$
$=6.023 \times 10^{19} / L$
$\because$ In $1000 \, mL$ number of hydroxyl ions
$=6.023 \times 10^{19}$
$\therefore $ In $100\, mL$ number of hydroxyl ions
$=\frac{6.023 \times 10^{19} \times 100}{1000}$
$=6.023 \times 10^{18} .$