Video Transcript
Scientists analyze a 140-gram
sample of contaminated river water. They find that the sample contains
105 grams of water. What is the impurity percentage for
this sample? (A) 7.5 percent, (B) 75 percent,
(C) 10 percent, (D) 2.5 percent, or (E) 25 percent.
We are told in the question that a
140-gram sample of contaminated river water contains 105 grams of water. So what makes up the remaining 35
grams of this sample? As we are told that the sample is
contaminated, the remaining 35 grams must be contaminants or impurities. Impurities are any undesired
material mixed in with the desired substance.
To answer the question, we need to
represent the amount of impurities in the sample with the impurity percentage. To calculate a standard percentage,
we divide the part in question by the whole then multiply by 100 percent. When we calculate an impurity
percentage, we want to know what part of the sample is made up of impurities. So the equation to calculate the
impurity percentage is impurity percentage equals the mass of the impurities divided
by the mass of the sample times 100 percent. The mass of the impurities is 35
grams. The total mass of the river water
sample is 140 grams. Performing the calculation, we find
that the impurity percentage is 25 percent.
So a 140-gram sample of river water
that contains 105 grams of water has an impurity percentage of 25 percent, answer
choice (E).
