Question

You are given a sample of a hydrate of CaSO4. The data collected and recorded in your laboratory notebook were: Mass of empty crucible 1.138 g Mass of crucible + sample before heating 1.457 g Mass of crucible + sample after heating 1.436 g Calculate the formula of the hydrate.

Answer 1

mass of sample = 1.457 g - 1.138 g

= 0.319 g

mass of sample after heating = 1.436 g - 1.138 g

= 0.298 g

mass of H2O = mass of hydrated salt - mass of anhydrous salt

mass of H2O = 0.319 g - 0.298 g

mass of H2O = 0.021 g

Molar mass of H2O,

MM = 2*MM(H) + 1*MM(O)

= 2*1.008 + 1*16.0

= 18.016 g/mol

mass(H2O)= 0.021 g

use:

number of mol of H2O,

n = mass of H2O/molar mass of H2O

=(2.1*10^-2 g)/(18.02 g/mol)

= 1.166*10^-3 mol

Molar mass of CaSO4,

MM = 1*MM(Ca) + 1*MM(S) + 4*MM(O)

= 1*40.08 + 1*32.07 + 4*16.0

= 136.15 g/mol

mass(CaSO4)= 0.298 g

use:

number of mol of CaSO4,

n = mass of CaSO4/molar mass of CaSO4

=(0.298 g)/(1.362*10^2 g/mol)

= 2.189*10^-3 mol

Let formula of hydrate be CaSO4.XH2O

use:

X = mol (H2O)/mol (CaSO4)

X = 1.166*10^-3 / 2.189*10^-3

X = 0.5

Answer: CaSO4 . 1/2 H2O