Question

A compound contains 0.643 grams hydrogen and 3.857 grams of carbon.

a) Find its empirical formula.

b) Find its molecular formula if its molecular weight is 56.0 g/mol.

Answer 1

Answer – We are given, mass of H = 0.643 g

Mass of C = 3.857 g

A) Empirical formula

First we need to calculate the moles of each given

We know formula

Moles = given mass / molar mass

Moles of H = 0.643 g / 1.00794 g.mol^-1

                     = 0.638 moles

Moles of C = 3.857 g / 12.0107 g.mol^-1

                     = 0.321 moles

Now moles of C is lowest, so need to divided each moles by number of moles of C

H = 0.638 / 0.321 = 2

C = 0.321 /0.321 = 1

S, empirical formula is CH₂

B) Now we are given molecular weight of molecular formula is 56.0 g/mol

Empirical formula mass = 12.0107 + 2x 1.0079

                                           = 14.0256 g

We know, molecular formula = n x empirical formula

So, n = molecular formula weight / empirical formula weight

n = 56 / 14.0256

= 4

So, molecular formula = 4 x CH₂

                                         = C₄H₈