The mole (a chemist's counting unit)
The concept of amount in moles is central to chemistry because it allows us to keep track of the number of molecules, atoms, or ions present in a given sample of a substance. One mole of a type of particle is equal to 6.022 × 1023 of that particle just like a dozen is equal to 12 particles.
The relationship between the amount in moles of a substance and the amount in moles of its atoms
The subscripts in the formula for a pure substance indicate
The relationship between amount in moles of anything and its mass
The molar mass (M) is the mass of one mole of substance and has the unit g mol–1.
The concept of amount in moles is central to chemistry because it allows us to keep track of the number of molecules, atoms, or ions present in a given sample of a substance. One mole of a type of particle is equal to 6.022 × 1023 of that particle just like a dozen is equal to 12 particles.
The relationship between the amount in moles of a substance and the amount in moles of its atoms
The subscripts in the formula for a pure substance indicate
- the relative numbers of the atoms in one formula unit of the substance, and
- the relative amounts in moles of those atoms in one mole of the substance.
1 molecule of H2O contains 2 atoms of H and 1 atom of O 6.02 × 1023 molecules of H2O contains 2 × 6.02 × 1023 atoms of H and 1 × 6.02 × 1023 atoms of O 1 mole of H2O contains 2 moles of H and 1 mole of O
The mathematical relationship between the amount in moles of a substance and the amount in moles of the atoms that comprise the substance is used in many chemical calculations. Using a mathematical relationship as shown below for C6H12O6 in these calculations avoids slips where the person doing the calculation makes the wrong choice between multiply and divide.
n(C6H12O6) = n(C) n(C6H12O6) = n(H) 6 12
If either amount in moles is known, the other can be calculated by substituting into this relationship.
The relationship between amount in moles of anything and its mass
The molar mass (M) is the mass of one mole of substance and has the unit g mol–1.
The molar mass for a substance can be calculated from its formula and the molar masses of the atoms comprising the substance.
The relative atomic masses appearing under the symbol for the element in the Periodic Table equal the molar mass of atoms of that element.
The relative atomic masses appearing under the symbol for the element in the Periodic Table equal the molar mass of atoms of that element.
The molar mass is a very important quantity because it allows the chemist to relate the mass (m) of a substance (which can be measured) to the amount in moles (n) (which cannot be measured).
The amount in moles present in a measured mass of a substance (n 
m) can be calculated by dividing by the mass of one mole of the substance (M).
The mass of a specified amount in moles of a substance (m n) can be calculated by multiplying by the molar mass (M).
m) can be calculated by dividing by the mass of one mole of the substance (M).| m = n × M |
n) can be calculated by multiplying by the molar mass (M).