JEE Main · 2021 · 25 Feb 2021 · Shift 1 · Practical and Analytical Chemistry

0.4 g mixture of

NaOH, Na_{2} CO_{3}

and some inert impurities was first titrated with

\frac{N}{10} HCl

using phenolphthalein as an indicator, 17.5 mL of HCl was required at the end point. After this methyl orange was added and titrated. 1.5 mL of same HCl was required for the next end point. The weight percentage of

Na_{2} CO_{3}

in the mixture is ____ (Rounded - off to the nearest integer)

Answer: 4

Solution

Phenolphthalein changes colour when the hydroxide has all been neutralised and the carbonate has been taken as far as bicarbonate. Methyl orange changes only when that bicarbonate is itself turned into carbonic acid, so the second stage measures one mole of acid for every mole of carbonate originally present:

n (N a_{2} C O_{3}) = \frac{0.1 \times 1.5}{1000} = 1.5 \times 1 0^{- 4} mol

m = 1.5 \times 1 0^{- 4} \times 106 = 0.0159 g

% = \frac{0.0159}{0.4} \times 100 = 3.98

The weight percentage of sodium carbonate in the mixture is 4.

JEE Main · 2021 · 25 Feb 2021 · Shift 1 · Integer answer

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JEE Main · 2021 · 25 Feb 2021 · Shift 1 · Practical and Analytical Chemistry

Solution

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