**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) HNO_{3}, (ii) C_{6}H_{12}O_{6}, (iii) C_{3}H_{8} (iv) C_{2}H_{5}OH (v) Al_{2}O_{3}, (vi) K_{2}Cr_{2}O_{7}

**Answer**

**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 H_{2}O is 18 amu.

**Steps for finding the formula mass**

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

Therefore, formula mass of H_{2}SO_{4} is 98 amu.

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

**HNO _{3}**

Step 1: The given compound is HNO_{3}.

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 HNO_{3} =** 63 amu.**

**C _{6}H_{12}O_{6}**

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 C_{6}H_{12}O_{6} = 72 + 12 + 96 = **180 amu**

**C _{3}H_{8}**

Masses of the atoms in the formula;

Carbon, C = 12 × 3 = 36 amu

Hydrogen, H = 1 × 8 = 8 amu

Therefore, formula mass of C_{3}H_{8} = 36 + 8 = **44 amu**

**C _{2}H_{5}OH**

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 C_{2}H_{5}OH = 24 + 5 + 16 + 1 = **46 amu**

**Al _{2}O_{3}**

Masses of atoms in the formula

Aluminum, Al = 2 × 27 = 54 amu

Oxygen, O = 3 × 16 = 48 amu

Therefore, formula mass of Al_{2}O_{3} = 54 + 48 = **102 amu**

**K _{2}Cr_{2}O_{7}**

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 K_{2}Cr_{2}O_{7} = 78 + 104 + 112 =**294 amu**

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