Question

$20mL$ of $0.1N$ $HCl$ is mixed with $20mL$ of$0.1N$ $KOH$. The $pH$ of the solution would be:
A: $0$
B: $7$
C: $2$
D: $9$

Answer 1

Hint: $pH$ is the measure of ${H^ + }$ions released by a substance when dissolved in water. On the basis of $pH$ it is defined whether the substance is acidic or basic. Species having $pH$ from $0$ to $7$are acidic and substances having $pH$ from $7$ to $14$ are basic in nature.
Formula used: $pH = - \log \left[ {{H^ + }} \right]$
Where $\left[ {{H^ + }} \right]$ is concentration of ${H^ + }$ ions, which will be given by the following formula.
$\left[ {{H^ + }} \right] = N \times V$ Or $\left[ {O{H^ - }} \right] = N \times V$
Where $N$ is normality and $V$ is volume

Complete answer:
In case of $HCl$ $\left[ {{H^ + }} \right] = 0.1 \times 20ml = 2mM$
In case of $KOH$ $\left[ {O{H^ - }} \right] = 0.1 \times 20ml = 2mM$
As seen there are no extra $\left[ {{H^ + }} \right]$ ions in the solution as they are completely neutralized by equal concentration of $\left[ {O{H^ - }} \right]$ coming from $KOH$. So our resulting solution will be neutral and we know neutral solution is having $pH = 7$

Therefore our answer to this question will be option B that is $7$ .

Additional information:
The concentration of a solution is also called molarity. Molarity is the measure of the number of moles present per liter of solution. It can be calculated as:
Molarity $ = \dfrac{{m \times 1000}}{{M \times V}}$ where $m$is given mass of substance, $M$ is molar mass of substance and $V$ is volume of solution. Units of molarity are$mol{L^{ - 1}}$ .
Normality $\left( N \right)$ is also known as the equivalent concentration of a solution. Normality is a measure of gram equivalent weight per liter of solution. The equivalent weight of an element is its gram atomic weight divided by its valence. Gram atomic weight is also known as atomic mass. The normality of a solution can be calculated as:
$Normality = \dfrac{m}{{E \times V}}$ Where$m$ weight of solute in grams, $E$ is equivalent weight of substance and $V$ is volume of solution in liter.

Note:
$pH$of any solution is the degree to measure whether the solution is acidic, basic or neutral.
For acidic solution $0 < pH < 7$
For neutral solution $pH = 7$
For basic solution $7 < pH < 14$