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JEE Main levelNumerical answerDifficulty 2 of 3mc172

A hydrated salt $Na X_{2} CO X_{3} \cdot x H X_{2} O$ contains $62.9%$ water of crystallisation by mass. Find $x$ to the nearest integer (Na = 23, C = 12, O = 16, H = 1).

Numerical answer type, tolerance 0.1.

Answer: 10

Let M = 106 + 18x. Then 18x/(106 + 18x) = 0.629, so 18x(1 - 0.629) = 0.629 x 106, giving x = 9.98. To the nearest integer x = 10 (Na2CO3.10H2O).

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