Exact Numbers

Only experimentally measured numbers or results calculated from them use significant digits.

Not every number is measured experimentally.  Some numbers are exact.  They have an infinite number of significant digits.  We usually consider the following kinds of numbers to be exact:

 

Example
counted numbers "Find the mass of 0.05 mole of calcium chloride."  The number 0.05 is considered to be an exact number (but the mass is not).

The subscripts in a formula, and the coefficients in a balanced chemical equation
CH4 (g)  + 2 O2 (g)  -----> CO2 (g)  + 2 H2O (g)
mathematical constants pi.gif (60 bytes) (The symbol pi.gif (60 bytes) is exact; however, the number 3.14 has only three significant figures, while 3.1416 has five.)
defined conversions 1 mL = 1/1000 L
1000 g = 1kg		  There are exactly 1000 mL in a Litre, by definition

When you use an exact number in a calculation, it has no effect on the number of significant digits you report in your final answer.

Which of the following would NOT be considered exact numbers?  Check all that apply.
a) the mass of an empty beaker
b) 1 metre = 100 cm
c) the width of the room is 10.0 m
d) there are 360 degrees in a circle