MCAT General Chemistry Question 13: Answer and Explanation

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Question: 13

13. The atomic weight of hydrogen is 1.008 amu. What is the percent composition of hydrogen by isotope, assuming that hydrogen's only isotopes are 1H and 2D?

  • A. 92% H, 8% D
  • B. 99.2% H, 0.8% D
  • C. 99.92% H, 0.08% D
  • D. 99.992% H, 0.008% D

Correct Answer: B

Explanation:

The easiest way to approach this problem is to set up a system of two algebraic equations, where H and D are the percentages of H (mass = 1 amu) and D (mass = 2 amu), respectively. Your setup should look like the following system:

H + D = 1 (percent H + percent D = 100%)

1 H + 2 D = 1.008 (atomic weight calculation)

Rearranging the first equation and substituting into the second yields (1 - D) + 2D = 1.008, or D = 0.008. 0.008 is 0.8%, so there is 0.8% D.

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