Question Video: Determining the Mass of a Substance Using a 4-Digit Mass Balance | Nagwa Question Video: Determining the Mass of a Substance Using a 4-Digit Mass Balance | Nagwa

# Question Video: Determining the Mass of a Substance Using a 4-Digit Mass Balance Chemistry

A student needs to use the mass balance apparatus to measure out a certain number of moles of a substance. Using a calculator, the student determines that they need to weigh out 12.108225 g of the substance. a) Why is this amount not a suitable value to measure using the mass balance apparatus? b) What mass should the student try to weigh out instead?

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### Video Transcript

A student needs to use the mass balance apparatus shown below to measure out a certain number of moles of a substance. Using a calculator, the student determines that they need to weigh out 12.108225 grams of the substance. Why is this amount not a suitable value to measure using the mass balance apparatus?

Well, the first thing we need to do is look at the mass balance apparatus. From the display, we can see it measures in grams. The student is looking out to weigh a mass in grams, which helps, but that’s not quite enough. The balance gives a value to a maximum of four decimal places. A gram mass balance accurate to four decimal places will be accurate to the nearest 0.0001 grams, one ten thousandths of a gram. But the mass the student calculated is to six decimal places. This is 100 times more precise than the balance is capable of.

The balance would be able to read the mass of 12.1082 grams. And if the student added a little bit more, it would be able to read 12.1083 grams. But the number produced by the student’s calculator is much more precise than the mass measurements this mass balance is capable of. So, this amount is not a suitable value to measure using the mass balance apparatus because the mass balance apparatus only measures to four decimal places.

What mass should the student try to weigh out instead? The number from the calculator was to six decimal places, but the balance only reads to four. So, to make sure we get as close to the calculated value as possible, we want to round that number to four decimal places. You may think you could just chop off the last two digits, but sometimes that won’t be as good as rounding. We know the first three decimal digits are one, zero, eight. And the digit in the fifth decimal place is a two. So, we round down. So, our final answer is 12.1082 grams.

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