Stoichiometry : It is calculation of masses or volumes of reactants and products involved in a chemically balanced reaction. Consider the formation of ammonia.

N₂ (g) + 3H₂ (g) <=>  2NH₃(g)

All are gases indicated by letter (g) and coefficients 3 for H2 and 2 for NH3 are called stoichiometric coefficients. The formation of ammonia can be interpreted in many ways:

• One mole of N₂(g) reacts with three moles of H₂(g) to give two moles of NH₃(g).
• 28g of N₂(g) reacts with 6g at H₂(g) to give 34g of NH3(g).
• 22.4L of N₂(g) reacts with 67.2L of H₂(g) to give 44.8L of NH₃(g)

The weight 7.52 gm of a substance indicates that it is reliable to the nearest hundredth of a gram and may be expressed as 7.52 ± 0.01. It means slightest variation may occur at the second place of decimal or we can say that uncertainty  is ± 0.01 g.

Now consider the weight 6.4234 g. It may correctly be expressed as 6.4234 ± 0.001 g.

In the first case the weight contains three significant figures and in the second case weight contains five significant figures.

1. Significance of zero : If zero is used to locate the decimal point it is not considered as significant figure. Thus in 0.0072 there are only two significant figures whereas in 70.40, there are four significant figures since zero is after 4. Again in 0.0070 there are two significant figures, since zero after 7 is significant for it has a meaning when written in exponentials. If we compare 7.0 × 10–3 and 7 × 10–3, the first term has uncertainty of one in seventy and second has uncertainty of one in seven. The exponential term does not add to number of significant figures.
2. Addition and subtraction of quantities : In this case the uncertainty in the result is equal to the sum of the uncertainties of the individual quantities.
3. Multiplication and division : In this case the uncertainty in the result is equal to the sum of the percentage of individual uncertainties.
4. Rounding off : The following rules are observed.
   1. If the digit after the last digit to be retained is less than 5, the last digit is retained as such. e.g. 1.752 = 1.75 (2 is less than 5).
   2. If the digit after the last digit to be retained is more than 5, the digit to be retained is increased by 1. e.g. 1.756 = 1.76 (6 is more than 5).
   3. If the digit after the last digit to be retained is equal to 5, the last digit is retained as such if it is even and increased by 1 if odd.
5. Calculations involving addition and subtraction : In case of addition and subtraction the final result should be reported to the same number of decimal places as the number with the minimum number of decimal places .
6. Calculations involving multiplication and division : In this case the final result should be reported having same number of significant digits as that of the number having least significant digits.