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 |
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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 |
|
mathematical constants | (The symbol 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.