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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.

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