Question

How many moles of $HCl$ must be removed from $1$ litre of aqueous $HCl$ solution to change its $pH$ from $2$ to $3$?

• A. $1$
• B. $0.02$
• C. $0.009$
• D. $0.01$

Answer 1

Answer: (C)

$0.009$

When $pH=2$, $\left[H^{+}\right]={10}^{-2}=0.01$ $M$

$Molarity=\frac{Moles}{Volume\left(Litres\right)}$

$⟹0.01=\frac{Moles}{1}=0.01$ $moles$

When $pH=3$, $\left[H^{+}\right]=0.001={10}^{-3}$ $M$

$⟹0.001=\frac{Moles}{1}=0.001$ $moles$

For $pH=3,$ there should be only $0.001$ $moles$ in the solution.

So, we have to remove $0.1-0.001$ $moles=0.009$ $moles$

Hence, option $C$ is correct.