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Question

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

A
7
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B
9
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C
12.75
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D
11.85
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Solution

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+HClNaCl+H2O

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

They react 1:1, so after reaction 1.41.17=0.23 mmol of NaOH is left. Final volume of the solution is 20+13=33 mL,
thus final concentration of NaOH left is 0.2333=0.0070 M.

pOH=log(0.0070)=2.15

pH=142.15=11.85

Hence, option D is correct.

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