Comments on: What are Significant Figures?

By: David Bradley

David Bradley — Sat, 10 Jul 2010 08:49:00 +0000

Yes, that really applies across the board doesn’t it? You simply cannot introduce precision that is not in the original measurements.

By: Dan Lednicer

Dan Lednicer — Fri, 09 Jul 2010 23:11:48 +0000

Formal analytical chemistry requires that the number of places used in every step in a chain calculation be cut back to the number of significant places delivered by the original measurement(weight, instrument reading etc…). The cut should not be left to the last step. Calculations carried out on hand calculators (which can be cut to the sigificant value) will often vary slightly from those using a spreadsheet (which are a pain to round at each step)..

By: David Bradley

David Bradley — Fri, 09 Jul 2010 07:14:58 +0000

@Anonymous It’s not. 7.89+5.876+3.1 = 16.866 but to 3 sig figs that 16.9. Dunno where you get 13.8 from…

By: Anonymous

Anonymous — Fri, 09 Jul 2010 06:28:46 +0000

hmm Example :7.89+5.876+3.1=13.866 but why is the answer is 13.8

By: Felix

Felix — Fri, 08 Jan 2010 10:09:22 +0000

Thanks, u saved my ass from my AS exams next week. This is the only thing I am loosing marks at :)

By: Harmon

Harmon — Mon, 12 Feb 2007 14:21:02 +0000

When I teach sig figs to my General Chemistry students, I advise them to delay rounding to the correct number of sig figs until the end of the calculation. This helps avoid introducing additional rounding errors. (Of course, carrying all 9 extra non-sig figs is overkill. One extra should do it.)

By: Lednicer

Lednicer — Fri, 09 Feb 2007 16:20:14 +0000

To add an extra complication most spreadsheet programs such as Lotus and Excel carry out calculations out to 14 (Lotus) or so decimal places no matter what shows on the screen. The Format command only modifies the display NOT the value being used. There is a way to limit the places used in calculations but it is a real pain as there is no global command; it must instead be applied individually to each value every time it appears.

This poses a real problem in multistep operations required to calculate analyses that must be submitted to regulatory authorities. Real differences in final numbers can be seen in values calculated by a good hand calculator and those obtained from a spreadsheet.