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ICSE Class 10 Chemistry Question 4 of 28

Stoichiometry — Percentage Composition — Empirical & Molecular Formula — Chemical Equations — Question 12

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Question 1(2000)

Washing soda has the formula Na2CO3.10H2O. What mass of anhydrous sodium carbonate is left when all the water of crystallization is expelled by heating 57.2 g of washing soda.

Answer

Na2CO3.10H2O→ΔNa2CO3+10H2O2(23)+12+3(16)2(23)+1210[2(1)+10[2(1)+16]+3(16)+16]=46+12+48=46+12=180 g+10(18)+48=286 g=106 g\begin{matrix} \text{Na}_2\text{CO}_3.10\text{H}_2\text{O} & \xrightarrow{\Delta} & \text{Na}_2\text{CO}_3 & + & 10\text{H}_2\text{O} \\ 2(23) + 12 + 3 (16) & & 2(23) + 12 & & 10[2(1) \\ + 10[2(1) + 16] & & + 3(16) & & + 16] \\ = 46 + 12 + 48 & & = 46 + 12 & & = 180 \text{ g} \\ + 10(18) & & + 48 \\ = 286 \text{ g} & & = 106 \text{ g} \\ \end{matrix}

286 g of washing soda had 106 g of anhydrous sodium carbonate

∴ 57.2 g will have = 106286\dfrac{106}{286} x 57.2 = 21.2 g anhydrous sodium carbonate.

Hence, 21.2 g anhydrous sodium carbonate is left.