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

• A. 22 g
• B. 100 g
• C. 178 g
• D. 46 g

Answer: (C)

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.