Question

$100mL$ of $0.2N$ $HCl$ is added to $100mL$ of $0.18N$ $NaOH$ and the whole volume is made one litre. The pH of the resultant solution is:

• A. $1$
• B. $2$
• C. $3$
• D. $4$

Answer 1

Answer: (C)

$3$

When you 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,

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

there are $100\times 0.2=20mmol$ of HCl and $100\times 0.18=18mmol$ of HCl.

Since they react 1:1, so after the reaction, 18mmol of $HCl$ reacts with 18 mmol of $NaOH$ to neutralize each other.

Only 2mmol of $HCl$ remains.

Final volume solution is $1000ml$

Now the concentration of $H^{+}$ ions will be $2/1000=0.002$

$pH=-log\left[H^{+}\right]=-log\left(0.002\right)=2.69$

$pH=2.69$