Chemistry Question about measurement

[mom31257]

If I use a digital scale that does not show any decimal places in the reading, should I consider that an exact measurement? Should I list the measurement to the 10th place with a zero?

 

For example, I weighed a large steel nut at 30 grams. Do I list the measurement as 30 or 30.0? I posted this in the Chemistry social group, but I need to know this morning and don't know if anyone will see it or not. Thanks!

[Brenda in MA]

Amy,

 

You cannot list any more significant figures than what the scale displays. Since the scale is showing you 30 grams, that means that the actual weight (assuming the scale is calibrated correctly) is between 29.5 g and 30.4 g, so you cannot list the weight as 30.0 grams (which would mean that the actual weight is between 29.95 and 30.04 g). You should list the weight as 30 grams.

 

Brenda

[mom31257]

Thanks, Brenda. I kept reading things about going one more decimal place than the instrument measures when you are using something you have to estimate such as a ruler or thermometer, so I wasn't sure if that applied to electronic equipment or not.

[Brenda in MA]

Yeah, that's right with something like a ruler where you can see how much past one line the measurement is, you can sometimes put one more decimal place, like you could probably estimate pretty well whether a reading was 30, 30.5, or 31, but going more "fine" than 0.5 is probably not a good idea in that case.

 

However, with a digital readout that shows 30, you have no way of knowing how close you are to 29.5 or to 30.4, so just putting the number as 30 is the best way to go.

 

Brenda

[Julie of KY]

As soon as you record 30.0, you are saying you have three significant figures and that your measurement is somewhere between 29.95 and 30.04.

[AngieW in Texas]

Technically, you should record the answer as 3.0 x 10¹ g or as .030 kg to show that you have two significant digits. If you just record the answer as 30 g, the number of significant digits is ambiguous.