A sample of hydrated magnesium sulfate (MgSO4)

has a mass of 12.845 g. When it is dehydrated,

the anhydrous sample has a mass of 6.273 g.

Which is the correct chemical formula for the

hydrate?

MgSO4.7H2O

Explanation:

Let the formula for the hydrated magnesium sulphate be MgSO4. xH2O

Mass of the hydrated salt (MgSO4. xH2O) = 12.845g

Mass of anhydrous salt (MgSO4) = 6.273g

Mass of water molecule (xH2O) = Mass of the hydrated salt - Mass of anhydrous salt = 12.845 - 6.273 = 6.572g

Now, we can obtain the number of mole of water molecule present in the hydrated salt as follows:

Molar Mass of hydrated salt (MgSO4. xH2O) = 24 + 32 + (16x4) + x (2 + 16) = 24 + 32 + 64 + x (18) = 120 + 18x

Mass of xH2O / Molar Mass of MgSO4. xH2O = Mass of water / mass of hydrated salt

18x/120 + 18x = 6.572/12.845

Cross multiply to express in linear form

18x x 12.845 = 6.572 (120 + 18x)

231.21x = 788.64 + 118.296x

Collect like terms

231.21x - 118.296x = 788.64

112.914x = 788.64

Divide both side by 112.914

x = 788.64 / 112.914

x = 7

Therefore the formula for the hydrated salt (MgSO4. xH2O) is MgSO4.7H2O