PSAT Math Grid-Ins Question 75: Answer and Explanation

Question: 75

The following table shows the chemical makeup of one mole (a unit of measure commonly used in chemistry) of acetone and the approximate mass of a mole of each component element.

Chemical Makeup of One Mole of Acetone
Element	Number of Moles	Mass per Mole (grams)
Oxygen	1	16
Carbon	3	12
Hydrogen	6	1

If a chemist starts with 1,800 grams of acetone and uses up 930 grams, approximately how many moles of carbon are left? Round your answer to the nearest whole mole

Correct Answer: 45

Explanation:

45

Difficulty: Hard

Strategic Advice: The question contains several steps. Think about the units given in the question and how you can use what you know to find what you need.

Getting to the Answer: Start with grams of acetone: the chemist starts with 1,800 and uses up 930, so there are 1,800 - 930 = 870 grams left. From the previous question, you know that 1 mole of acetone has a mass of 58 grams, so there are moles of acetone left. Don't grid in this amount because you're not finished yet! The question asks for the number of moles of carbon, not acetone. According to the table, each mole of acetone contains 3 moles of carbon, so there are 15 × 3 = 45 moles of carbon left. Grid in 45.