Burets are used to measure volumes of liquids when higher accuracy is required. The
calibration lines on a 50-mL buret represent 0.1-mL increments, allowing us to make
estimates to within 0.02 mL (^15 of one calibration increment) with reasonable certainty
(Figure 1-15b). Experienced individuals estimate volumes in 50-mL burets to 0.01 mL
with considerable reproducibility. For example, using a 50-mL buret, we can measure out
38.57 mL (four significant figures) of liquid with reasonable accuracy.
Accuracy refers to how closely a measured value agrees with the correct value.
Precisionrefers to how closely individual measurements agree with one another. Ideally,
all measurements should be both accurate and precise. Measurements may be quite pre-
cise yet quite inaccurate because of some systematic error,which is an error repeated in
each measurement. (A faulty balance, for example, might produce a systematic error.) Very
accurate measurements are seldom imprecise.
Measurements are frequently repeated to improve accuracy and precision. Average
values obtained from several measurements are usually more reliable than individual
measurements. Significant figures indicate how precisely measurements have been made
(assuming the person who made the measurements was competent).
Some simple rules govern the use of significant figures.
1.Nonzero digits are always significant.For example, 38.57 mL has four significant figures; 288 g has three significant figures.
2.Zeroes are sometimes significant, and sometimes they are not.
a. Zeroes at the beginningof a number (used just to position the decimal point)
are never significant.For example, 0.052 g has two significant figures; 0.00364 m has three significant figures.
These could also be reported in scientific notation (Appendix A) as 5.2 10 ^2 g and
3.64 10 ^3 m, respectively.
b.Zeroes betweennonzero digits are always significant.For example, 2007 g has four significant figures; 6.08 km has three significant figures.
c. Zeroes at the endof a number that contains a decimal point are always sig-
nificant.For example, 38.0 cm has three significant figures; 440.0 m has four significant figures.
These could also be reported as 3.80 101 cm and 4.400 102 m, respectively.
d.Zeroes at the endof a number that does not contain a decimal point may or
may not be significant.1-8 Use of Numbers 23