Question on: JAMB Chemistry - 2024

The empirical formula of an organic liquid hydrocarbon is XY. If the relative molar masses of X and Y are 72 and 6 respectively, its vapour density is likely to be

A

33

B

66

C

39

D

78

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Correct Option: C

Here's how to solve this problem:

  1. Determine the ratio of molar masses: The molar mass ratio of X to Y is 72:6, which simplifies to 12:1.
  2. Calculate the molecular formula: Since the empirical formula is XY, the molecule contains 1 part X and 1 part Y. The molar mass of XY is (1 * 72) + (1 * 6) = 78. To find the molecular formula, additional information like vapor density is required.
  3. Calculate molar mass using vapor density: Vapor density (VD) is half the molar mass (M). Therefore, M = 2 * VD.
  4. Finding the correct answer: Using the options to check, if VD = 33 (Option A), then M = 66; if VD = 66 (Option B), then M = 132; if VD = 39 (Option C), then M = 78. The vapor density most consistent with the molecular formula XY is likely 39.

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