# Question Video: Calculating the Impurity Percentage of a Sample of River Water Chemistry

Scientists analyze a 140 g sample of contaminated river water. They find that the sample contains 105 g of water. What is the impurity percentage for this sample?

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### 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).