It is determined that the remaining mass of unknown bromide salt (XBr3) is 2.84g. Evaluate the water of crystallization (y).

Question

In order to determine the water of crystallization (y) of the unknown bromide salt,

3.70g hydrated unknown bromide salt (XBr₃ . yH₂O) was placed into 105^oC oven for

2 days to remove all water of crystallization in the compound following the

chemical equation below.

XBr₃ . yH₂O(s) → XBr₃(s) + yH₂O(g)

It is determined that the remaining mass of unknown bromide salt (XBr3) is 2.84g.

Evaluate the water of crystallization (y).

Answer 1

The water lost is the key, and it gives y = 0.0168 × M(XBr₃). With indium as X, that comes to y = 6.

Step 1 — mass of water driven off

Everything lost during heating was water:

m(H₂O) = 3.70 g − 2.84 g = 0.86 g

n(H₂O) = 0.86 / 18.02 = 0.0477 mol

Step 2 — moles of anhydrous salt

n(XBr₃) = 2.84 / M, where M = M(X) + 3(79.90) = M(X) + 239.71 g/mol

Step 3 — y is the mole ratio

y = n(H₂O) / n(XBr₃) = 0.0477 / (2.84 / M) = 0.0168 × M

You need the identity of X to finish

The problem as posted does not say what X is, and without its molar mass y cannot be pinned down. That was almost certainly established in an earlier part of the question — check back for it and substitute into the formula above.

You can also work backwards, because y must be a whole number (a hydrate has a fixed integer number of waters per formula unit). Testing candidates:

AlBr₃ (266.7): y = 4.48
CrBr₃ (291.7): y = 4.90
FeBr₃ (295.6): y = 4.97
GaBr₃ (309.4): y = 5.20
InBr₃ (354.5): y = 5.96, i.e. 6
YBr₃ (328.6): y = 5.52

Only indium lands within 1% of an integer, so the salt is very likely InBr₃·6H₂O. Confirm against whatever part (a) told you.

Cross-check with percent water

Measured: (0.86 / 3.70) × 100 = 23.2% water

Predicted for InBr₃·6H₂O: 6(18.02) / [354.5 + 6(18.02)] = 108.1 / 462.6 = 23.4% ✓

Agreement to within 0.2 percentage points is strong support for that formula.

Two practical notes on this experiment

1. Heat to constant mass. Two days at 105 °C is generous, but the real criterion is reweighing until the mass stops changing — residual water makes y come out low.
2. Cool in a desiccator before weighing. A hot anhydrous salt reabsorbs atmospheric moisture quickly, which also biases y low. If your y lands just under an integer, this is usually why.