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Question

4.10 g of hydrated magnesium sulphate crystals MgSO4．xH2O　gave 2.00 g of anhydrous salt on careful heating. Calculate the formula of the hydrated salt.
Find x..
(molar mass : Mg:24 ; S:32 ; O:16 ; H:1)

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Answer 1

Molar mass of MgSO4 = 24 + 32 + 4x16 = 120 g
Molar mass of H2O = 2x1 + 16 = 18 g

In 4.10 g of MgSO4•xH2O :
Mass of MgSO4 = 2.00 g
Mass of H2O = 4.10 - 2.00 = 2.10 g

Since no. of moles of a substance = (mass / molar mass)
Mole ratio MgSO4 : H2O
= (2.00/120) : (2.10/18)
= 0.0167 : 0.117
≈ 1 : 7

But in MgSO4•xH2O, mole ratio MgSO4 : xH2O = 1 : x

Hence, x = 7

Answer 2

MgSO4．xH2O　----> MgSO4 + xH2O　
mole ratio of MgSO4．xH2O　to MgSO4 = 1:1
=> mole of MgSO4．xH2O　 = mole of MgSO4
4.1/( 24+32+16*4+18x ) = 2/(24+32+16*4)
246 = 120+18x
x=7