Chemistry 100Significant Figures

Rules for Significant Figures

Zeros used to locate decimal points are NOT significant. e.g., 0.5 kg = 5. X 102 g 1 sig. fig.

What if the measurement was 500 g?

5 x 102 g 1 sig. fig. 5.0 x 102 g 2 sig. figs. 5.00 x 102 g 3 sig. figs.

Certain values are exact (e.g., from definitions) 1 kg = 1000.00000……… g exactly 1 s = 1000.0000 …………. ms exactly 1 cal = 4.18400000 ……… J exactly.

Addition and Subtraction Operations.

Results from +, - operations should be reported to the same number of decimal places as that of the least precise term. 12.37+ 10.7 = 23.07 (calculator)

12.37 2 places after the decimal. 10.7 1 place after the decimal.

Final answer must have only 1 place after the decimal = 23.1

Multiplication and Division Operations

Results from x, operations should be rounded off to the same number of significant figures possessed by the least precise term. 0.80 x 217.81 = 174.248 0.80 2 sig. figs. 217.81 5 sig. figs. Final answer can only possess 2 sig.

figs. = 1.7 x 102

Carrying digits through a long calculation

Example:129 / 13.76 + 10.1 = 9.375 + 10.1 =

19.475 = 19.5 Usually carry an extra digit through

the calculations, then round off the final answer according to the significant figure rules.