While researching a topic, a student has taken the following notes:
● Element abundance is a relative measure of the occurrence of a chemical element in a given environment.
● An element's relative abundance can be expressed as a mass fraction, mole fraction, or volume fraction.
● Mass fraction is the ratio of an element’s mass to the combined mass of all elements in a given environment.
● The mass fraction of calcium (Ca) is 17,100 parts per million (ppm) or 1.71% on Earth.
● The mass fraction of potassium (K) is 160 ppm, or 0.016%, on Earth.
The student wants to specify an element’s relative abundance on Earth. Which choice most effectively uses relevant information from the notes to accomplish this goal?
Explanation
Answer: B) Expressed as a mass fraction, the relative abundance of potassium on Earth is 16,000 ppm or 1.60%.
Goal: Specify an element's relative abundance on Earth.
Why? Specifying abundance requires an element plus its measurement. B gives potassium's exact abundance: 16,000 ppm (1.60%)—specific and complete.
Traps:
- A) About expression methods → no specific abundance
- C) Says "can be expressed" → no actual value
- D) Comparison → doesn't specify abundance