Question

What is the pH of the resulting solution when equal volumes of $0.1$ M NaOH and $0.01$ M HCl are mixed?
A) $7.0$
B) $1.04$
C) $12.65$
D) $2.0$

Answer 1

In the given question, NaOH is a basic compound and HCl is an acidic compound and the concentration of NaOH is more than the concentration of HCl. This means that the resultant mixture would be basic in nature i.e. the pH value will be more than or equal to $7$

Complete step by step solution:
The concentration of NaOH is given to us as $0.1$ M.
The concentration of HCl is given to us as $0.01$ M.
Here, the concentration of NaOH is more than the concentration of HCl. This means that $O{H^ - }$ ions will be more in number when compared to ${H^ + }$ ions.
When the concentration of $O{H^ - }$ ions is more than the concentration of ${H^ + }$ ions, the mixture will be basic in nature.
Now, we can calculate the concentration of $O{H^ - }$ ions as follows.
$\dfrac{{0.1 - 0.01}}{2} = 0.045M$
Hence the concentration of $O{H^ - }$ ions is calculated to be $0.045M$
From this concentration value, we can now calculate the pOH value by using antilog. Let us write the formula for pOH.
$pOH = - \log \left[ {O{H^ - }} \right]$
Let us substitute the concentration of $O{H^ - }$ ions in the above equation. We get $- \log 0.045 = 1.35$
We know that the sum of pH and pOH is $14$. We can now substitute the value of pOH in this relation to get the pH value.
$pH + 1.35 = 14$
On solving, we get $pH = 14 - 1.35 = 12.65$

Therefore, the pH value of the given mixture is $12.65$ i.e. option C.

Note: It is to be noted that the pH value of a neutral mixture is $7$. So, mixtures or solutions having pH more than $7$ are said to be basic in nature and those having pH less than $7$ are said to be acidic in nature. In the above solution, the pH of the mixture is $12.65$ meaning that it is basic in nature.