GRE Subject Test: Chemistry : Calibrations and Precision

Study concepts, example questions & explanations for GRE Subject Test: Chemistry

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Example Questions

Example Question #1 : Analytical Chemistry

Michael buys several bags of balloons. On the package, it says that each bag has 100 balloons. He opens the bags and only one of them has 100 balloons inside; the other bags either have too many or too few.

How would you describe the bag of balloons with 100 balloons inside?

Possible Answers:

Accurate, but not precise

Neither accurate nor precise

Precise, but not accurate

Accurate, but the precision cannot be determined

Both accurate and precise

Correct answer:

Accurate, but not precise

Explanation:

This bag is accurate because it provided the correct number of balloons, however, the process is not precise as the results were clearly not repeatable.

Accuracy deals with how close the measurement got to the accepted measurement. Precision deals with how consistent the measurement is. The bag with 100 balloons inside matched the claim made on the bag, meaning it was accurate. It was not precise because the other measurements show that the number of balloons is variable.

Example Question #271 : Gre Subject Test: Chemistry

Michael's scale measures the mass of objects as consistently  less than their actual mass. How would you describe the scale?

Possible Answers:

Accuracy and precision are synonyms

It is both accurate and precise

It is accurate, but not precise

It is precise, but not accurate

It is neither accurate nor precise

Correct answer:

It is precise, but not accurate

Explanation:

Precision measures is how consistently a device records the same answer. In this case, Michael's scale is ALWAYS  short. Even though it displays the wrong value, it is consistent. That means it is precise. Measuring a object will always display a mass of ; the results are easily reproduced.

Accuracy is how well a device measures something against its accepted value. In this case, Michael's scale is not accurate because it is always off by .

Example Question #1 : Understanding Accuracy And Precision

An brand of fruit snacks claims that each bag of fruit snacks has a mass of . After weighing three bags, Wally observes the masses to be , , and  .

How can Wally describe the accuracy and precision of the first bag he measured?

Possible Answers:

There are insufficient data points to draw a conclusion

Accurate, but not precise

Accurate and precise

Precise, but not accurate

Neither accurate nor precise

Correct answer:

Accurate, but not precise

Explanation:

The claim for the mass of the first bag is accurate; the brand says there should be in each bag and there was in the first bag.

The claim on the first bag is not precise, as the results are not replicated universally throughout the experiment. The masses of the bags fluctuate, with the average of all three bags equal to .

Example Question #2 : Analytical Chemistry

A bowman is shooting arrows at a target. Which of the following demonstrates high accuracy but low precision?

Possible Answers:

The bowman consistently hits around the target but never hits the bullseye

The bowman consistently hits to the right of the bullseye

The bowman consistently misses the target and hits a tree in the same spot

The bowman consistently hits the bullseye

The bowman consistently hits to the left of the target

Correct answer:

The bowman consistently hits around the target but never hits the bullseye

Explanation:

Accuracy is measured as the degree of closeness to the actual measurement. In our case, accurate shots will hit the bullseye. Precision is measured as the degree of closeness of one measurement to the next. In our case, precise shots will be clustered together.

To get high accuracy but low precision, measurements must center around the target value but be variable. The bowman's arrows will not be clustered (low precision), but will be accurately distributed around the bullseye. If all the shots were averaged, the bullseye would be at the center.

Example Question #3 : Analytical Chemistry

Which of these is an example of high precision?

Possible Answers:

An archer hits the same spot on the target three times in a row

A student tries to throw a pencil into the garbage can and makes it in

An archer hits the bulls-eye

A student correctly calculates the mass of an object to be 54kg

A student correctly calculates the acceleration due to gravity to be 

Correct answer:

An archer hits the same spot on the target three times in a row

Explanation:

Precision is a measure of reproducibility. If multiple trials produce the same result each time with minimal deviation, then the experiment has high precision. This is true even if the results are not true to the theoretical predictions; an experiment can have high precision with low accuracy.

In contrast, accuracy is the measure of difference between a calculated value and the true value of a measurement. High accuracy demands that the experimental result be equal to the theoretical result.

An archer hitting a bulls-eye is an example of high accuracy, while an archer hitting the same spot on the bulls-eye three times would be an example of high precision.

Example Question #4 : Analytical Chemistry

Which of these is an example of high accuracy?

Possible Answers:

An archer hits the same spot on the target three times in a row

A student repeatedly calculates the mass of an object to be 49 grams, when the true mass is 52 grams

A student calculates the acceleration due to gravity to be  using three different methods

A student throws three pencils into the trash can, but misses and hits the window each time

An archer hits the bulls-eye

Correct answer:

An archer hits the bulls-eye

Explanation:

Accuracy is the measure of difference between a calculated value and the true value of a measurement. High accuracy demands that the experimental result be equal to the theoretical result.

In contrast, precision is a measure of reproducibility. If multiple trials produce the same result each time with minimal deviation, then the experiment has high precision. This is true even if the results are not true to the theoretical predictions; an experiment can have high precision with low accuracy.

An archer hitting a bulls-eye is an example of high accuracy, while an archer hitting the same spot on the bulls-eye three times would be an example of high precision.

Example Question #1 : Calibrations And Precision

What is the molar concentration of a 2% by mass  solution with a density of

Possible Answers:

Correct answer:

Explanation:

By definition, 2% by mass means 2 grams of  in every 100 grams of solution. In order to solve this problem, we may assume we are dealing with 100 grams of solution. We can use the density as a conversion factor to determine the volume of solution we are dealing with:

In order to calculate the number of moles of we need to convert 2 grams of  to moles using it's molecular weight. This is because we are assuming we are dealing 100 grams of a 2%  solution. 

In order to calculate molarity, we need to plug the moles and volumes calculated into the following equation:

Example Question #2 : Calibrations And Precision

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Using the calibration curve given, if given the absorbance of 1.51, what would be the concentration?

Possible Answers:

Correct answer:

Explanation:

A calibration curve is a graph that gives the relationship between a measured quantity and an analytical signal. In this question the analytical signal is absorbance and the measured quantity is the concentration. To answer this problem, simply determine where the absorbance value 1.5 is on the y-axis and find what point the x-axis crosses this absorbance.

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