Question

A hydrated calcium phosphate contains 7.14 % water of crystallization the molecular mass of
phosphate is 252. The percentage composition is 15.18 % Ca, 24.6% P, 4% H and 57.11 % Oxygen
what is the molecular formula of the compound?

Answer 1

Percentage composition of Ca = 15.18%

percentage composition of P = 24.6 %

percentage composition of H = 4%

percentage composition of O = 57.11%

we know,

atomic mass of Ca = 40 amu

atomic mass of P = 31 amu

atomic mass of H = 1 amu

atomic mass of O = 16 amu

find percentage - mass ratio of each atom.

for Ca = 15.18/40 = 0.3795 ≈ 0.4

for P = 24.6/31 = 0.793548387 ≈ 0.8

for H = 4/1 = 4

for O = 57.11/16 = 3.569375 ≈ 3.6

now, simple ratio of percentage-mass ratio,

Ca : P : H : O = 0.4 : 0.8 : 4 : 3.6 = 1 : 2 : 10 : 9

so, empirical formula of phosphate is $CaP_2H_{10}O_9$

now molecular mass of phosphate is 252. and also 7.14 % of 252 is water in crystal form.

so, mass of water = 7.14 % of 252

≈ 18

means, one mole of water is included in compound.[ because molar mass of water is 18g/mol]

so, empirical formula is converted into $CaP_2H_8O_8.H_2O$

so, n × empirical mass = molecular mass

or, n × (40 + 31 × 2 + 10 × 1 + 16 × 9) = 252

or, n × (40 + 62 + 10 + 144) = 252

or, n × 256 = 252

or, n = 0.98 ≈ 1

so, molecular formula of phosphate is $CaP_2H_8O_8.(H_2O)$