Question

If "Epsom salt," MgSO4 x H2O is heated to 250°C. all the water of hydration is lost. On heating a 1.687- g sample of the hydrate, 0.824 g of MgSO4 remains. What is the formula of Epsom salt?

Answer 1

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

mass of H2O = 1.687 g - 0.824 g

mass of H2O = 0.863 g

Molar mass of H2O,

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

= 2*1.008 + 1*16.0

= 18.016 g/mol

mass(H2O)= 0.863 g

use:

number of mol of H2O,

n = mass of H2O/molar mass of H2O

=(0.863 g)/(18.02 g/mol)

= 4.79*10^-2 mol

Molar mass of MgSO4,

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

= 1*24.31 + 1*32.07 + 4*16.0

= 120.38 g/mol

mass(MgSO4)= 0.824 g

use:

number of mol of MgSO4,

n = mass of MgSO4/molar mass of MgSO4

=(0.824 g)/(1.204*10^2 g/mol)

= 6.845*10^-3 mol

use:

X = mol (H2O)/mol (MgSO4)

X = 4.79*10^-2 / 6.845*10^-3

X = 7

Answer: 7