A sample of a hydrate of NiCl2 (.235g) is heated to drive off water leaving .128g. What is X in NiCl2(XH2O)?

What is the formula of the hydrate?

Answer:
0.235 - 0.128 = 0.107 g H2O
molecular weight H2O = 18 g/mol
0.107 / 18 = 0.00594 moles H2O
molecular weight NiCl2 = 129.6 g/mol
0.128 g / 129.6 = 0.000987 moles NiCl2
the ratio between NiCl2 and H2O is 0.00594 / 0.000987 = 6
X is 6 and the formula is NiCl2 . 6 H2O
Formula of the hydrate is NiCl2.6H2O
X = 6

Solution
1. Get the number of moles water evaporated.

moles H2O = (0.235g - 0.128g) / 18.0 g/mol
moles H2O = 5.94 x 10^-3 mol

2. Get moles of NiCls

mole NiCl2 = 0.128g / 129.6 g/mol
mole NiCl2 = 9.88 x 10^-4

3. To get X, divide mole H2O evaported by mole NiCl2 left.

X = 5.94 x 10^-3 / 9.88 x 10^-4
X ~ 6

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