Question

5.72 grams of hydrated sodium carbonate is heating strongly.The white residue left over is 2.12 g.Calculate:
i.)Molecular mass of hydrated sodium carbonate.
ii.)The percentage of carbon in the compound.
Na=23,C=12,O=16,H=1
Pls solve this question with explanation

Answer 1

Hydrated formula = Na2CO3 . xH20

Mass of hydrated sodium carbonate = 5.72  

Mass of residue = 2.12 g

Mass of water = 5.72 - 2.12 = 3.6 g

As,  

n = m / M

n = 3.6 / 18 = 0.2 mol

Anhydrous sodium carbonate = 2.12 / 105.98 = 0.02 mol

Whole ratio will be :

Na2CO3 = 0.02 / 0.02 = 1

xH2O = 0.2 / 0.02 = 10

Hence , the formula will be Na2CO3 . 10 H2O

Molar mass of hydrated sodium carbonate will be = 286

Mass %age= Mass percent of C/Molar mass of Na2CO3 .10 H2O x 100

                     = 12 / 286 x 100 = 4.19 %