A problem many chemistry students face is calculating the number of representative particles in a substance. A substance has a definite chemical composition with a corresponding chemical formula. Representative particles can be atoms, molecules, formula units or ions, depending on the nature of the substance. The standard unit used to represent the amount of a substance is the mole, where 1 mole contains 6.02 x 10^23 particles. This quantity is referred to as Avogadro's number.
Measure the mass of the substance in grams. For example, you weigh a sample of water and its mass is 36.0 grams.
Calculate the molar mass of the substance by adding the average atomic masses of the individual atoms in the chemical formula. The average atomic mass for each element can be found in the Periodic Table. For example, the molar mass for water would be 18.0 grams per mole. Water is made of two hydrogen atoms, that each weigh 1.0 gram, and one oxygen atom, that weighs 16.0 grams.
Divide the mass measured in Step 1 by the molar mass determined in Step 2. This will change the unit of the substance to moles. Following the example, 36.0 grams / 18.0 grams/mole = 2 moles of water.
Multiply the value obtained in Step 3 by Avogadro's number, which represents the number of representative particles in a mole. Avogadro's number has a value of 6.02 x 10^23. Continuing the example, 2 moles of water * 6.02 x 10^23 particles per mole = 1.20 x 10^24 particles.
TL;DR (Too Long; Didn't Read)
The number of significant digits used in molar mass calculation and in calculating the number of representative particles depends on the number of significant digits in which the mass is measured. The number of significant digits in the answer to a calculation cannot exceed the number of significant digits in the mass measurement. If you are given the number of moles of a substance, then you only have to complete Step 4 in order to calculate representative particles.