Question

1. The molar mass of a compound is 126 g/mol, and it's empirical formula is C2H2O. What is compound's molecular formula?

2. A compound consist of 26.18g Si and 29.82g O. Calculate its empirical formula?

3. Calculate the empirical formula of the compound that consist of 70.4%F and 29.6%O.

Answer 1

1.

Empirical formula of the compound = C2H2O.

We know the following atomic weights

C = 12.0 g/mol

H = 1.0 g/mol

O = 16.0 g/mol

Hence, the mass of one empirical formula unit can be calculated as follows:

Now, it is given that the molar mass of the compound is 126 g/mol.

Hence, the number of empirical formula units in the molecular formula can be calculated as

Hence, there are 3 empirical units on the molecular formula. Hence, the molecular formula can be calculated as follows:

.

Hence, the molecular formula of the compound is C₆H₆O₃.

2.

The compound consists of 26.18 g of Si.

Atomic mass of Si = 28.0855 g/mol

Hence, the number of moles of Si in the compound is

The compound also consists of 29.82 g of O.

Atomic mass of O = 16.00 g/mol

Hence, the number of moles of O in the compound is

Hence, the ratio of moles of Si and O can be calculated as

Hence, there are twice as many O in the compound for every atom of Si.

Hence, the empirical formula must be .

3.

Let the mass of the compound be 100 g.

It is given that the compound is 70.4 % F by mass.

Hence, 70.4 g of the compound is F.

Atomic mass of F = 18.998 g/mol

Hence, the number of moles of F in 100 g of the compound is

Similarly, the compound is 29.6% O.

Hence, there are 29.6 g of O in 100 g of the compound.

Atomic mass of O = 16.0 g/mol

Hence, the number of moles of O in 100 g of the compound is

Hence, the mole ratio of F and O can be calculated as

Hence, there are twice as many F than O in the formula of the compound.

Hence, the empirical formula of the compound is .