Question

The $pH$ is $1$. The amount of $NaOH$ to be added to $100ml$ of such a $HCl$ solution to get $pH$ of $7$ is:

• A. $4g$
• B. $0.4g$
• C. $4mg$
• D. $0.4mg$

Answer 1

The correct option is B $0.4g$

To get $pH=7$ from $pH=1$, there must be complete neutralization.

$\therefore \left[H^{+}\right]=\left[O{H}^{-}\right]={10}^{-1}M$

$NaOH\to N{a}^{+}+O{H}^{-}$

$\left[O{H}^{-}\right]=\frac{W{t}_{NaOH}\times 1000}{M_{NaOH}\times 100}={10}^{-1}$

$\therefore \frac{Wt\times 10}{40}={10}^{-1}$

$\Rightarrow$Wt. of $NaOH$ required $=0.4g$