Question

What is the mass of water in 0.01132 moles of MgSO4.7H2O? (How can I do this with dimensional Analysis?)

Answer 1

Look up the molar masses of each component element:

• Mg : 24.305 g/mol

• S : 32.06 g/mol

• O : 15.999 g/mol

• H : 1.008 g/mol

Then MgSO₄•7H₂O has a total molar mass of about 246.466 g/mol. 7H₂O has a total molar mass of about 126.105 g/mol, so it makes up about 0.5117 or 51.17% of the total mass.

Now, 0.01132 mol of MgSO₄•7H₂O has a mass of

(0.01132 mol MgSO₄•7H₂O) × (246.466 g/mol) ≈ 2.79 g

which means water makes up about 1.428 g (i.e. 51.17% of the mass of MgSO₄•7H₂O).