Question

A molecular compound contains 92.3% carbon and 7.7% hydrogen by mass. What is the empirical formula of this compound? If 0.592 mol of the compound weighs 30.78g, what is its molecular formula?

Answer 1

Solution :-

assume mass of sample is 100 g

then mass of C = 92.3 g and mass of H = 7.7 g

lets calculate the moles of C and H

moles of C= mass/ molar mass

                 = 92.3 g / 12.01 g per mol

                 = 7.685 mol

moles of H= 7.7 g / 1.0079 g per mol = 7.639 mol H

now lets calculate the ratio

C= 7.685 mol / 7.639 mol = 1

H= 7.639 mol / 7.639 mol = 1

so the empirical formual = CH

now lets calculate the molar mass

molar mass = 30.78 g * 1 mol / 0.592 mol = 52 g per mol

emprical formula mass CH= 12.01 + 1.0079 = 13.02 g

ratio of the molar mass to empirical formula mass N = 52.0 g / 13.02 g = 4

so the molecular formula = N*empirical formula

                                        = 4*CH

                                        = C4H4

so the molecualr formula is C4H4