84 3. The student heated a white crystalline substance until all the crystal wat

Question

84 3. The student heated a white crystalline substance until all the crystal water is removed. The following experimental data is obtained by the student: The mass of the beaker and the watch glass: The mass of the beaker + watch glass + sample 57.880 g 59.993 g he mass of the beaker + watch glass + sample after first heating: 59.710 g The mass of the beaker + watch glass + sample after second heating: 59.700 g The mass of the beaker + watch glass + sample after third heating: 59.697 g Identify the unknown hydrate. The possible unknowns are: BaCl2.2H20 MgSO4.7H20 CuSO4.5H20 ZnSO4.7H20

Explanation / Answer

Mass of Sample = 59.993 - 57.880 = 2.113 g

Mass of Sample after 1st Heating = 59.710 - 57.880 = 1.83 g

Mass of Water Removed = 2.113 - 1.83 = 0.283 g

Mass of Sample after 2nd Heating = 59.7 - 57.88 = 1.82 g

Mass of Water Removed = 1.83 - 1.82 = 0.01 g

Total Mass of Water lost = 0.293 g

Moles of Water lost = 0.293 / 18 = 0.01628

Mass of Anhydrate = 1.82 g

Now check the molar mass of each hydrate:

CuSO4.5H2O = 159.61 g/mol

ZnSO4.7H2O = 287.58 g/mol

BaCl2.2H2O = 244.26 g/mol

MgSO4.7H2O = 246.47 g/mol

Now, Check the mole ratio of Anhydrous Salt : Water to get the required stoichiometric coefficient of H2O.

For BaCl2.2H2O,

Answer: BaCl2.2H2O

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