Chemistry · Some Basic Concepts Of Chemistry · NEET
An exact number is a number you get by counting whole objects or by a fixed definition, not by measuring with an instrument. Examples: 2 balls, 20 eggs, 6 students, or 1 metre = 100 cm. Because there is no measuring, there is no doubt in the value. It is perfectly exact.
Significant figures show how much doubt is in a value. An exact number has zero doubt. So you can imagine writing endless zeros after the decimal point: 2 = 2.000000... and 20 = 20.000000... Every one of those zeros is 'known for certain', so the number has infinite significant figures. NCERT states this exact rule.
No. This is the key point for NEET. When you multiply or divide, your answer keeps only as many significant figures as the measured value with the FEWEST significant figures. An exact number has infinite sig figs, so it is never the smallest. It NEVER limits your answer. Only your measured numbers decide the final sig figs.
No. Anything read off an instrument (ruler, burette, balance) is a measured number and always has some uncertainty. 11.2 mL means 11 is certain and the last 2 is uncertain (about plus or minus 1). Measured numbers have a limited, countable number of significant figures. Only counted whole objects and definitions are exact.
It depends on where it comes from. If 100 is a definition or a count (100 cm in 1 m, or 100 students counted), it is exact with infinite sig figs. But if 100 is a measured value like '100 g on a balance', it is NOT exact and its sig figs must be judged by the zero rules. Context decides.
Yes, defined conversion factors are exact. For example 1 kg = 1000 g, 1 dozen = 12, 1 hour = 60 minutes. These are fixed by definition, not measured, so they have infinite significant figures and do not reduce the sig figs of your final answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Infinite. '5 balls' is a counted, exact number. You can write it as 5.000000..., so it has infinite significant figures and never limits a calculation.
Since the 2019 SI redefinition, Avogadro's number is a defined exact value. For NEET, treat the value you are given (6.022 x 10^23) by its written significant figures in calculations, but know that the underlying definition is exact.
Usually exact numbers are written plainly (2, 20, 12). In scientific notation, remember a separate NCERT rule: all digits shown are significant, e.g. 4.01 x 10^2 has three significant figures.
In mole and stoichiometry problems, coefficients from balanced equations and unit conversions are exact. If you wrongly count their sig figs, you may round your final answer to too few digits and pick the wrong option. Knowing they are infinite keeps your answer's precision correct.
