Question

A sample of a compound containing boron (B) and hydrogen (H) contains 1.152 g of B and 0.3222 g of H. The molar mass of the compound is about 30 g. What is its molecular formula?

Answer 1

Solution :-

lets first calculate the moles of the B and H

moles = mass / molar mass

moles of B= 1.152 g / 10.811 g per mol = 0.1066 mol B

moles of H =0.3222 g / 1.0079 g per mol = 0.3197 mol H

now lets find the ratio of the moles

B= 0.1066 / 0.1066 = 1

H = 0.3197 /0.1066 = 3.00

therefore empirical formula = BH3

empirical formula mass of BH3 = 13.8 g per mol

now lets find the ratio of the molar mass to empirical formula mass

N= molar mass / empirical formula mass

N= 30.0 g / 13.8 = 2.17 rounded to 2

Therefore molecular formula = N*empirical formula

                                       = 2*BH3

                                       = B2H6

Hence molecular formula = B2H6