April 20, 2014

Homework Help: Chemistry

Posted by Summer on Thursday, September 14, 2006 at 9:32pm.

How does Significant Figures work?

If i had a number: 1,007,000 is that with 4 sig figs or 7 sig figs?

I checked 5 different resources and gotten conflicting answers.


The answer will depend on what's being measured and how it's being measured.
If that number were the amount of shares of common stock of some public company that were traded today then it could be 7 significant figures. On the other hand, if it were the number of miles some object might be from the earth then there might be only 4 significant figures. If it's some kind of measurement then most likely it has only 4 sig. figures. If it's something that can be counted, and the process is extremely accurate and verifiable, then there could be more than four signif. figures.
So the short answer depends.

It is hard to tell. If there was a period at the end, such as 1,007,000. then the answer is 7 sig figures. But with no period, we don't know if the zeros are sig or not. GENERALLY, I think the rule is not to count them if there is no period. In such practice, the number would be written as 1.007E+6. Then there would be no confusion and we would count 1, 0, 0, and 7 as sig figures.

The reason you get conflicting answers is this:
The notation for significant zeros is changing. The IBM keyboard is largely to blame. Years ago, trailing significant zeros could be labeled with an overscore (a line above the number) to indicate it was significant. We cant make overscores on the IBM keyboard. So panic has ensued.
Years ago, if your number had a significant zeros, the least significant zero had an overscore (I can type it with an underscore 1,007,000, (now the number has six significant digits), but no overscore. For awhile, bold was used to indicated the least significant zero (1,007,000, but that faded quickly. So now there is no method except shifting to scientific notation (decimals to the right of numbers and decimals are significant, ie 1.00700 is seven sig digits), or being told initially.
This above applies to measurements: as Roger pointed out, counting numbers, or defined numbers,(100years in a century) are infinitely precise. If 1007000 had been counted, it does not have seven significant digits, it has infinite significant digits..It cannot determine the precision of your final outcome. This confusion based on the disruption of typesetting by the IBM keyboard in computers in the last 20 years is probably the reason you got five different answers. The most common rules widely adhered to are here:

(Broken Link Removed)

There are only three rules cited there.

I remember just 17 years ago, texts listed as many as seven rules.

For Further Reading

Thanks for this refresher Bob. It's been awhile since I looked at signif. figs. I wasn't aware that the short-comings of the keyboard were partly to blame for the confusion here. I do remember using the bar, but I couldn't remember what the rules were for it. I think some sets of rules also tell how to make calculations with measurements and determine sig. figs., that might account for the different rule counts.
In any case -if you read this Summer- always look at what your text states and know what the instructor expects. You should be alright if you do that.

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