CHEMISTRY

JAMB 2011 - Question 43

Chemistry 2011 JAMB Past Questions - Question 43: An organic compound has an empirical formula  CH2O and vapour density of What is its molecular formula?

Choose the correct answers from the options given.
An organic compound has an empirical formula  CH2O and vapour density of What is its molecular formula?


A:
B:
C:
D:
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Correct Answer

C

Explanation

To find the molecular formula of the organic compound with the empirical formula CH?O and a vapor density of x, we need to compare the molar mass of the empirical formula with the molar mass of the actual compound.

1. Calculate the molar mass of the empirical formula CH?O:
  - Carbon (C) has a molar mass of approximately 12.01 g/mol.
  - Hydrogen (H) has a molar mass of approximately 1.008 g/mol.
  - Oxygen (O) has a molar mass of approximately 16.00 g/mol.

  So, the molar mass of CH?O = (12.01 * 1) + (1.008 * 2) + (16.00 * 1) ≈ 12.01 + 2.016 + 16.00 ≈ 30.026 g/mol.

2. Determine the empirical formula mass:
  The empirical formula mass of CH?O is 30.026 g/mol.

3. Use the relationship between empirical formula mass and molar mass to find the molecular formula:
  Molar mass = n * empirical formula mass,
  where n is a whole number representing the ratio of the molar mass to the empirical formula mass.

4. Given the vapor density, we know that the molar mass of the compound is 2 times the empirical formula mass (since vapor density is the ratio of the molar mass of a compound to the molar mass of hydrogen, and hydrogen has a molar mass of approximately 2 g/mol).

  So, the molecular formula mass = 2 * empirical formula mass.

  Molecular formula mass = 2 * 30.026 g/mol ≈ 60.052 g/mol.

5. Now, we find the molecular formula that has a molar mass close to 60.052 g/mol.

Let's calculate the molar mass of the options:

- C?H?O: (12.01 * 2) + (1.008 * 5) + (16.00 * 1) ≈ 24.02 + 5.04 + 16.00 ≈ 45.06 g/mol
- C?H?O?: (12.01 * 3) + (1.008 * 6) + (16.00 * 3) ≈ 36.03 + 6.048 + 48.00 ≈ 90.078 g/mol
- C?H?O: (12.01 * 2) + (1.008 * 4) + (16.00 * 1) ≈ 24.02 + 4.032 + 16.00 ≈ 44.052 g/mol

Comparing the calculated molecular formula mass (60.052 g/mol) with the molar masses of the options, we see that C?H?O is the closest match. Therefore, the molecular formula of the compound is C?H?O.