A 5.00 g mixture of anhydrous Li2CO3 and Na2CO3 is heated strongly until constant mass. Only Li2CO3 decomposes (to Li2O and CO2) and the mass falls by 1.00 g. Taking Li 7, Na 23, C 12, O 16, find the mass percentage of Li2CO3 in the mixture, to the nearest integer.

JEE Advanced levelNumerical answerDifficulty 3 of 3sb083

A 5.00 g mixture of anhydrous $Li X_{2} CO X_{3}$ and $Na X_{2} CO X_{3}$ is heated strongly until constant mass. Only $Li X_{2} CO X_{3}$ decomposes (to $Li X_{2} O$ and $CO X_{2}$ ) and the mass falls by 1.00 g. Taking Li 7, Na 23, C 12, O 16, find the mass percentage of $Li X_{2} CO X_{3}$ in the mixture, to the nearest integer.

Numerical answer type, answer in %, tolerance 0.5.

Only

Li X_{2} CO X_{3} Li X_{2} O + CO X_{2}

occurs, since Na2CO3 is stable. Moles of CO2 lost

= 1.00/44 = 0.02273

, equal to moles of Li2CO3, whose mass is

0.02273 \times 74 = 1.682

g. Percentage

= 1.682/5.00 \times 100 = 33.6 \approx 34