MCAT General Chemistry Question 13: Answer and Explanation
Home > MCAT Test > MCAT general chemistry practice tests
Test Information
- Use your browser's back button to return to your test results.
- Do more MCAT general chemistry practice tests.
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.