Question

Find the pH of $300ml$ of $0.3M{\text{ HCl}}$ + $200ml$ of $0.2M{\text{ NaOH}}$?

Answer 1

Hint: The symbol pH is derived from word potenz which means power, that’s why pH means power of hydrogen and pH is defined as the negative logarithm to base 10 of the activity of hydrogen ion, i.e. ${a_{{H^ + }}}$.
At low concentrations, the activity of hydrogen ions is numerically equal to its molarity represented as $[{H^ + }]$.

Complete answer:
The pH scale ranges from 1 to 14 while a pH of 7 is considered neutral whereas a pH less than 7 corresponds to an acid and a pH greater than seven is considered basic.
An acid and a base react together to give salt and water as products, this reaction is called neutralization reaction.
Step 1: Calculation of moles-
$300ml$ of $0.3M$ of $HCl$ gives
$\dfrac{{300 \times 0.3}}{{1000}} = 0.09moles$
Similarly, $200ml$ of $0.2M$ of $NaOH$ gives
$\dfrac{{200 \times 0.2}}{{1000}} = 0.04moles$
Step 2: Finding the limiting reagent-
$NaOH + HCl \to NaCl + {H_2}O$
From the above equation :
One mole of hydrochloric acid reacts with one mole of sodium hydroxide to produce one mole of sodium chloride. So, $0.04$ mole of NaOH will react with $0.04$ moles of HCl. Therefore, $0.05$ mole of HCl will be left unreacted.
Step 3: Calculating the concentration of ${H^ + }$ion-
The concentration of ${H^ + }$ ion in $500ml$ of solution= $\dfrac{{0.05 \times 1000}}{{500}} = 0.1$
Step 4: Calculating the pH:
 $pH = - \log [{H^ + }]$
Substituting the given values,
$pH = - \log [0.1]$
$pH = 1$

Note:
${K_a}$ is called dissociation or ionization constant of an acid and it may be defined as the fraction of the total number of molecules of an electrolyte (acid or base) which ionizes into ions. Dissociation constants are used to compare the strengths of acids and bases. The acid (or a base) having a higher value of dissociation constant is stronger.
Stronger acid will have higher ${K_a}$ value but smaller $p{K_a}$ value while a weaker acid will have small ${K_a}$ value but higher $p{K_a}$ value.