Question

An organic substance containing carbon, hydrogen and oxygen gave the following percentage composition. C = 40.7%; H = 5.1% and O = 54.2% Calculate the empirical formula weight of the compound. (At.wt of C = 12, O = 16, H = 1)

• A. 118
• B. 59
• C. 177
• D. 133

Answer 1

Answer: (B)

59

Answer 2

To calculate the empirical formula weight:

1. Assume 100 g sample: Mass of C = 40.7 g, H = 5.1 g, O = 54.2 g

2. Calculate moles:

   • Moles of C = 40.7/12 ≈ 3.39
   • Moles of H = 5.1/1 = 5.1
   • Moles of O = 54.2/16 ≈ 3.39

3. Find the mole ratio:

   $C : H : O \approx 3.39 : 5.1 : 3.39 \Rightarrow 1 : 1.5 : 1$

4. Multiply by 2 to obtain whole numbers:

   $C_2H_3O_2$

5. Calculate the empirical formula weight:

   $(2 \times 12) + (3 \times 1) + (2 \times 16) = 24 + 3 + 32 = 59$