Question

When 3.94g of hydrated copper(II) sulfate crystals were heated, 2.52g of the anhydrous salt remained, calculate the formula of the hydrated salt?

Answer 1

The formula of the hydrate is `"CuSO"_4·"5H"_2"O"`.

Explanation:

Step 1. Calculate the mass of water in the sample

`"Mass of water = mass of hydrate - mass of anhydrous salt"`
`"= 3.94 g - 2.52 g = 1.42 g"`

Step 2. Calculate the mass of water that combines with 1 mol of `"CuSO"_4`

The molar mass of `"CuSO"_4` is 159.61 g/mol.

`"Mass of water" = 159.61 color(red)(cancel(color(black)("g CuSO"_4))) × ("1.42 g H"_2"O")/(2.52 color(red)(cancel(color(black)("g CuSO"_4)))) = "89.93 g H"_2"O"`

Step 3. Calculate the moles of `"H"_2"O"`

`"Moles of water" = 89.93 color(red)(cancel(color(black)("g H"_2"O"))) × ("1 mol H"_2"O")/(18.02 color(red)(cancel(color(black)("g H"_2"O")))) = "4.99 mol H"_2"O"`
`≈ "5 mol H"_2"O"`

Step 4. Determine the formula of the hydrate

1 mol of `"CuSO"_4` combines with 5 mol of `"H"_2"O"`.

∴ The formula of the hydrate is `"CuSO"_4·"5H"_2"O"`.