Question 3: (a) What is formula mass of a compound? What are the steps involved in calculating the formula mass of a compound? Explain with an example.

(b) Calculate formula mass of the following compounds. (i) HNO3, (ii) C6H12O6, (iii) C3H8 (iv) C2H5OH (v) Al2O3, (vi) K2Cr2O7

## Definition of formula mass

Formula mass is the sum of the atomic masses of the atoms present in the formula of a compound.

For example, formula mass of H2O is 18 amu.

## Steps for finding the formula mass

1. Write the formula of the compound; say H2SO4.
2. Identify the number of atoms of different elements in the compound. Here Hydrogen, Sulpher and Oxygen.
3. Calculate the masses of the individual atoms in the formula. Here, H = 2 × 1 = 2 amu, S = 32 amu, O = 4 × 16 = 64 amu.
4.  Find the sum of the masses of the atoms in the formula. Here it is, 2 + 32 + 64 = 98 amu.

Therefore, formula mass of H2SO4 is 98 amu.

(b) To find the formula masses of the given compounds.

HNO3

Step 1:  The given compound is HNO3.

Step 2: Atoms present in the formula are, Hydrogen (H), Nitrogen (N) and Oxygen (O).

Step 3: Atomic masses of the atoms present in the formula are;

Hydrogen = 1 × 1 = 1 amu

Nitrogen = 1 × 14 = 14 amu

Oxygen = 3 × 16 = 48 amu

Step 4: Sum of the masses of the atoms in the formula, 1 + 14 + 48 = 63

Therefore, formula mass of HNO3 = 63 amu.

C6H12O6

Masses of the atoms in the formula;

Carbon, C = 6 × 12 = 72 amu

Hydrogen, H = 12 × 1 = 12 amu

Oxygen, O = 6 × 16 = 96 amu

Therefore, formula mass of C6H12O6 = 72 + 12 + 96 = 180 amu

C3H8

Masses of the atoms in the formula;

Carbon, C = 12 × 3 = 36 amu

Hydrogen, H = 1 × 8 = 8 amu

Therefore, formula mass of C3H8 = 36 + 8 = 44 amu

C2H5OH

Masses of the atoms in the formula;

Carbon, C = 2 × 12 = 24 amu

Hydrogen, H = 5 × 1 = 5 amu

Oxygen, O = 1 × 16 = 16 amu

Hydrogen, H = 1 × 1 = 1 amu

Therefore, formula mass of C2H5OH = 24 + 5 + 16 + 1 = 46 amu

Al2O3

Masses of atoms in the formula

Aluminum, Al = 2 × 27 = 54 amu

Oxygen, O = 3 × 16 = 48 amu

Therefore, formula mass of Al2O3 = 54 + 48 = 102 amu

K2Cr2O7

Masses of the atoms in the formula,

Potassium, K = 2 × 39 = 78 amu

Chromium, Cr = 2 × 52 = 104 amu

Oxygen, O = 7 × 16 = 112 amu

Therefore, formula mass of K2Cr2O7 = 78 + 104 + 112 =294 amu