Question Video: Describing the Accuracy and Precision of Mass Measurements Chemistry

Ten students are measuring mass with a malfunctioning balance. The malfunctioning balance always displays values that are 40% too large. The students determine a total of twenty mass values for a single heap of caustic soda. All of their mass measurements are between 100.1 and 100.3 grams. What combination of words should be used to describe their measurements?

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Video Transcript

10 students are measuring mass with a malfunctioning balance. The malfunctioning balance always displays values that are 40 percent too large. The students determine a total of 20 mass values for a single heap of caustic soda. All of their mass measurements are between 100.1 and 100.3 grams. What combination of words should be used to describe their measurements? (A) Imprecise and inaccurate, (B) precise and inaccurate, (C) imprecise and accurate, (D) precise and accurate.

To answer this question, we need to understand the difference between precision and accuracy. First, let’s get rid of the answer choices to get some screen space. Accuracy is how close a scientific measurement is to the correct value. Precision is how close scientific measurements are to each other. A dartboard is often used to explain the difference between these two terms. The center of the dartboard represents the correct value. If our aim is accurate and we throw a dart at the dartboard, the dart will hit the center circle of the board. But if our aim is not accurate, the dart will miss the center of the board.

Now let’s say we throw three darts at the dartboard, one right after the other. One of them hits the target, but the rest are scattered across the dartboard. Because these darts did not land close to each other, our dart throwing was not precise. So we throw another three darts to try to redeem ourselves, which all land in one corner of the dartboard. Because these three darts are close together, this time our dart throwing was precise. Notice that these three darts that were precise are nowhere near the center of the dartboard. So even though our dart throwing was precise, it wasn’t accurate because the darts didn’t hit the center of the board representing the correct value.

Our dart throwing might also be accurate but imprecise if the darts all hit the center circle of the dartboard, but they aren’t close together. Of course, the goal is to be both accurate and precise; that is, we would like to have all of our darts hit the center of the dartboard and be close together.

Now that we know the difference between accuracy and precision, let’s return to the question. And the question tells us that students are measuring mass with a malfunctioning balance that displays values that are 40 percent too large. If the balance isn’t displaying the correct value, there’s no way that measurements made with this balance can be accurate. Next, the question tells us that the students measure the mass of a heap of caustic soda 20 times. All 20 of the measurements are between 100.1 and 100.3 grams. 20 measurements all within 0.2 grams of each other are very close together, so these measurements are very precise.

So “Which combination of words should be used to describe their measurements?” That is answer choice (B) precise and inaccurate.