Question

The pH of a solution made by mixing $50mL$ of $02M$ $HCl$ and $75mL$ of $0.1M$ $NaOH$ is:
A. 3
B. 13
C. 5
D. 1.7

Answer 1

pH is defined as the negative logarithm of ${H^ + }$ ion concentration. It is justified as the power of hydrogen. Further, a pH scale is a tool used for measuring acids and bases. The strong bases are completely ionized in the solution so, the concentration of the hydroxide ion is the same as that of the base concentration.

Formula used:
pH$= - \log [{H^ + }]$

Complete step by step answer:
The pH is a measure of the concentration of hydrogen ions, the acidity or alkalinity of a solution. The pH scale is normally between 0 to 14. Further, the aqueous solutions with a pH less than 7are acidic and with pH greater than 7 are basic or alkaline.
However, on mixing a strong acid and a strong base, neutralization (pH$= 7$ takes place and the resulting solution can be either an acid or a base depending on the concentration.
Now, we are given a solution of $HCl$ and $NaOH$ so, let’s find its pH.
The reaction between the given compounds is as shown:
$HCl + NaOH \to NaCl + {H_2}O$
Now, from the given equation we can calculate the remaining concentration of hydrogen ions.
Now, $7.5mL$ of $NaOH$ reacts with $7.5mL$ of $HCl$ leaving behind $2.5mL$ of $HCl$
Therefore, $[{H^ + }] = \dfrac{{2.5}}{{125}}$
$= 2 \times {10^{ - 2}}$
According to the given formula,
pH$= - \log [{H^ + }]$
$= 2 - \log 2$ ($\log 2$Is$0.3010$)
= 1.7

Hence, option D is correct.

Note: pH is a function of acidic or basic water content. Water with more free hydrogen ions is acidic and water with more free hydroxyl ions is basic. Moreover, humans have a typical pH range from 7.35 to 7.45. Our stomach has a pH range of around 1.5 to 3.5 , which further makes it acidic.