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
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 H2O is 18 amu.
Steps for finding the formula mass
- Write the formula of the compound; say H2SO4.
- 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 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
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