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27 July, 22:38

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.

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  1. 27 July, 23:26
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    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
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