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General rules for Significant Figures

reporting results

Results are generally reported with one uncertain significant figure (usually +/-1). Example: reporting a mass of 12.554 g indicates that the experimenter is certain that the true mass lies in the range 12.553 - 12.555 g. If the uncertainty in the last significant figure is greater than +/- 1 it should be written explicitly: 12.554 g +/- 0.003 g.

addition and subtraction

The answer must be truncated to the same least significant decimal place as the number with the least significant figure from the right.

Example:

  8.9444 g
+18.52   g
--------
 27.46   g

multiplication and division

The answer has the same number of significant figures as the factor with the smallest number of significant figures.

Example:

8.9 g / 12.01 g/mol = 0.74 mol
It is common to carry one additional significant figure through extended calculations and to round off the final answer at the end.

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