An organic compound has an empirical formula CH2O and vapour density of 45. What is the molecular formula? [C = 12, H = 1, O = 16]

Analysis

To find the molecular formula of an organic compound, we need to determine the relationship between its empirical formula and its molecular mass. We are given the empirical formula ($CH_2O$) and the vapour density of the compound.

Concept

1. Molecular Mass: The molecular mass of a substance is twice its vapour density. $$\text{Molecular Mass} = 2 \times \text{Vapour Density}$$
2. Relationship: The molecular formula is a multiple of the empirical formula. $$\text{Molecular Formula} = (\text{Empirical Formula})_n$$Where$n = \frac{\text{Molecular Mass}}{\text{Empirical Formula Mass}}$

Step-by-Step Solution

Step 1: Calculate the molecular mass. Given the vapour density is 45:

Step 2: Calculate the empirical formula mass. Using the atomic masses provided ($C=12, H=1, O=16$):

Step 3: Determine the value of $n$.

Step 4: Determine the molecular formula. Multiply the empirical formula ($CH_2O$) by $n=3$:

Conclusion

Comparing our result to the given options, $C_3H_6O_3$ corresponds to option C.

Exam Tip

Always remember that Vapour Density is a shortcut to finding the molar mass of a gas or volatile liquid. If you ever forget the formula, just remember that the molecular mass of Hydrogen gas ($H_2$) is 2, and its vapour density is 1; this ratio ($2:1$) applies to all gases!