Question

Calculate the equivalent mass of $\text{N}{{\text{a}}_{2}}\text{C}{{\text{O}}_{3}}$ when it is titrated against HCl in the presence of phenolphthalein.
A. 106
B. 53
C. 26.5
D. 212

Answer 1

Hint: We can say that the equivalent mass of compound is the total number of gram equivalents of the compound. Equivalent mass can be calculated by finding the ratio of molar mass of the substance to the acidity or basicity or n-factor of the same substance. Here sodium bicarbonate is titrated with HCl. So, we will have to consider the n-factor of the sodium bicarbonate. From the formula, we can clearly say that 1 molecule of carbonate will react with 2 molecules of sodium to form the sodium bicarbonate.

Complete step by step answer:
Sodium carbonate is titrated with hydrochloric acid to know the concentration of sodium carbonate with the help of a known concentration of hydrochloric acid.
It means that the fixed mass of an element can displace 1 part by mass of hydrogen, 35.5 parts by mass of chlorine and 8 parts by mass oxygen.
To calculate the equivalent mass of sodium carbonate firstly we will write the chemical equation:

\[\text{N}{{\text{a}}_{2}}\text{C}{{\text{O}}_{3}}\text{ + HCl }\to \text{ NaCl + NaHC}{{\text{O}}_{3}}\]

Now, we will calculate the molar mass of sodium carbonate:
$\text{N}{{\text{a}}_{2}}\text{C}{{\text{O}}_{3}}$
= $\left( \text{23 }\times \text{ 2} \right)\text{ + 12 + }\left( \text{16 }\times \text{ 3} \right)$
= $\text{46 + 12 + 48}$
= $\text{106}$
The formula to find the equivalent mass of sodium carbonate is:

Equivalent mass = $\dfrac{{Molar\;mass}}{{Number\; of\;hydrogens\;added\;or\;removed}}$

$\text{= }\dfrac{106}{1}\text{ }=\text{ 106}$
So, the equivalent mass of sodium carbonate is 106.

Therefore, option A is the correct answer.

Note: Equivalent mass has great use in the volumetric analysis because those compounds who have more equivalent mass are preferred over those compounds who have less equivalent mass. After all, the chance of error in weighing is very much less.