ACS General Chemistry 2 Practice Exam

What is the mass of ethanol in the 200 g sample described above (89% by mass ethanol)?

22 g 100 g 178 g 46 g

Correct Answer:

178 g

Explanation:

Percent by mass tells you what portion of the total mass is ethanol. To find the mass of ethanol, multiply the total sample mass by the fraction that is ethanol: 0.89 × 200 g = 178 g. So the ethanol mass in the sample is 178 g. The other numbers would correspond to smaller fractions (for example, 22 g is about 11% of 200 g, 100 g is 50%, and 46 g is about 23%), which don’t match the given 89% by mass.

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