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.
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
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