Question

What is pH of mixture prepared by mixing 20 mL 0.07 M NaOH and 13 mL 0.09 M HCl ?

• A. $7$
• B. $9$
• C. $12.75$
• D. $11.85$

Answer 1

The correct option is D $11.85$

When we mix strong acid and strong base they react till one of the reagents runs out. What is left is responsible for pH.

Reaction taking place in the solution is :

$NaOH+HCl\to NaCl+H_{2}O$

Initially, there were $20\times 0.07=1.4$ mmol of NaOH and $13\times 0.09=1.17$ mmol of HCl.

They react 1:1, so after reaction $1.4-1.17=0.23$ mmol of NaOH is left. Final volume of the solution is $20+13=33$ mL,

thus final concentration of NaOH left is $\frac{0.23}{33}=0.0070$ M.

$pOH=-\log \left(0.0070\right)=2.15$

$pH=14-2.15=11.85$

Hence, option D is correct.