(i) Calculate the number of gram atoms in 4.6 grams of sodium (Na = 23).
(ii) Calculate the percentage of water of crystallization in CuSO4.5H2O (H = 1, O = 16, S = 32, Cu = 64)
(iii) A compound of X and Y has the empirical formula XY2. Its vapour density is equal to its empirical formula weight. Determine its molecular formula.
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(i) 23 g of Na = 1 g. atom
∴ 4.6 g. of Na = $\frac{1}{23}$ x 4.6 = 0.2 g
Hence, 0.2 g. atoms
(ii) Molecular weight of hydrated copper sulphate CuSO4.5H2O = 64 + 32 + 4(16) + 5[2(1) + 16]
= 64 + 32 + 64 + 5[18]
= 64 + 32 + 64 + 90
= 250 g
250 g of CuSO4.5H2O contains 90 g of water of crystallization.
∴ Percentage of water of crystallization = $\frac{90}{250}$ x 100 = 36%
Hence, percentage of water of crystallization is 36%
(iii) Empirical formula = XY2
Empirical formula weight = V.D.
Molecular weight = 2 x V.D.
$$\begin{aligned}\text{n} = \frac{\text{Molecular weight}}{\text{Empirical formula weight}} \\[0.5em] \text{n} = \frac{\text{2 x V.D.}}{\text{V.D.}} \\[0.5em] \text{n} = 2\end{aligned}$$
∴ Molecular formula = n[E.F.] = 2[XY2] = X2Y4