(4) Empirical Formula (10 points) Rose-red crystals of CoClh hydrate were crysta
ID: 704411 • Letter: #
Question
(4) Empirical Formula (10 points) Rose-red crystals of CoClh hydrate were crystallized from aqueous solution and 3.61 g were isolated. Moderate warming of this product yielded 2.51 g of violet-blue crystals. Further heating produced a sky-blue powder that weighed 1.97 g and melted at 735°C (i.e., an anhydrous salt). The weight that was lost was all found to be water (18.02 g/mole ). What is the composition of the rose-red crystals and the blue-violet material assuming that the sky-b powder was anhydrous Cocl (129.8 g/mole)Explanation / Answer
The composition of the rose-red crystals is CoCl2.6H2O
And the composition of the blue-violet material is CoCl2.2H2O
Explanation:
The number of moles of sky-blue powder (CoCl2) = 1.97 g/129.8 g mol-1 = 0.015177 mol
The number of moles of blue-violet material and that of rose-red crystals should also be 0.015177 mol.
The molecular weight of blue-violet material = 2.51 g/0.015177 mol = 165.4 g/mol
= 129.8 + 2*18.02
Therefore, the blue-violet material is CoCl2.2H2O
The molecular weight of rose-red crystals = 3.61 g/0.015177 mol = 237.9 g/mol
= 129.8 + 6*18.02
Therefore, the blue-violet material is CoCl2.6H2O
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