Question

A student prepares a solution by dissolving 1.000 mol of Na2SO4 in water. She accidentally leaves the container uncovered and comes back the next week to find only a white, solid residue. The mass of the residue is 322.2 g. Determine the chemical formula of this residue.

Answer 1

Answer:

Formula is NA2SO4•10H2O

Sodium sulfate decahydrate.

Explanation:

Sodium sulfate is soluble in water. Soduim sulphare form various hydrates, so if the solution is open to the atmosphere for a week at least a lot of the water will have evaporated leaving behind a solid hydrate of Soduim sulfate.

Mass of anhydrous Na2SO4 = molar mass * number of moles

Molar mass = (23*2) + (32*1) + (16*4)

= 146 g/mol

Mass = 146*1

= 146 g of NA2SO4

NA2SO4•nH2O --> Na2SO4 + nH2O

Molar mass of hydrate, NA2SO4.nH2O

= (142 + 18n) g/mol

Mass of NA2SO4.nH2O = (142 + 18n)*1

= (142 + 18n) g

Mass of the residue = 322.2 g

Therefore, 142 + 18n = 322.2

18n = 180.2

n = 10

Formula is NA2SO4•10H2O

Sodium sulfate decahydrate.

0.0588 mol MgSO4 / 0.0588 = 1 mol MgSO4

0.412 mol H2O / 0.0588 = 7 mol H2O

Formula is MgSO4•7H2O