Question

It is often possible to change a hydrate into an anhydrous compound by heating it to drive off the water (dehydration). A 31.38 gram sample of a hydrate of MnSO4 was heated thoroughly in a porcelain crucible, until its weight remained constant. After heating, 18.29 grams of the anhydrous compound remained. What is the formula of the hydrate?

Answer 1

Let the formula for hydrate be:

MnSO4.XH2O

mass of H2O = mass of hydrated salt - mass of anhydrous salt

mass of H2O = 31.38 g - 18.29 g

mass of H2O = 13.09 g

Molar mass of H2O,

MM = 2*MM(H) + 1*MM(O)

= 2*1.008 + 1*16.0

= 18.016 g/mol

mass(H2O)= 13.09 g

use:

number of mol of H2O,

n = mass of H2O/molar mass of H2O

=(13.09 g)/(18.02 g/mol)

= 0.7266 mol

Molar mass of MnSO4,

MM = 1*MM(Mn) + 1*MM(S) + 4*MM(O)

= 1*54.94 + 1*32.07 + 4*16.0

= 151.01 g/mol

mass(MnSO4)= 18.29 g

use:

number of mol of MnSO4,

n = mass of MnSO4/molar mass of MnSO4

=(18.29 g)/(1.51*10^2 g/mol)

= 0.1211 mol

use:

X = mol (H2O)/mol (MnSO4)

X = 0.7266 / 0.1211

X = 6

Answer: MnSO4.6H2O