Significant Figures With Rules in Chemistry

Significant Figures

What are Significant Figures?

The reliable digits in a number that are known with the certainty are called significant figures. The last digit of measured number is generally considered uncertain by ± 1.

For Example : 
0.0112g contains three S.F and uncertainty is ± 0.0001 mg.
11/2mg contains three S.F and uncetainty is ± 0.1mg.

The uncertainty and the number of S.F do not change when measurement is expressed in sub units.

For Example:
0.0112g = 11.2mg in 0.0112g uncertainty is ± 0.0001g and in 11.2 mg uncertainty is ± 0.1mg as 0.0001g = 0.1mg. Therefore uncertainty and number of S.F do not change.

Number Of Significant Figures (Digits) Depends Upon :

  1. The accurancy of the measuring instrument.
  2. The size of the object to be measured.
In any series of measurements, the measured numbers are obtained to a certain degree of precision. The precision of measured is indicated by the number of significant figures.
  • Precision : How close a series of measurements are to each other.
  • Precise : Consistent (Reliability)
  • Accuracy : How close a measurement is to the accepted value.
  • Accurate : Correct.

Rules For Determining Significant Figures

  1. All non zero digits are considered significant : For example 123.45 have five significant figures = 1,2,3,4 and 5.
  2. Zero between two non zero digits are significant B/C these are obtained through actual measurement : For example 101.12 have five significant figures 1,0,1,1 and 2.
  3. Zeros locating the decimal point in a number less than one are not significant figures : For example 0.00012 has two significant figures 1 and 2.
  4. Final Zero to the right of the decimal point are significant figures b/c they ruke out the possiblity of any other digit : For example 12.2300 have six significant figures 1,2,2,3,0 and 0.
  5. Zeros that locate the decimal point in a number larger than one are not necessarily significant : For example 40cm have one S.F 4.
A number with all zero digits (e.g 0.000) has no significant digits, because the uncertainty is larger than the actual measurement.

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