# Relative formula mass and moles

- Relative formula mass (Mr) is the sum of the relative atomic masses of all atoms in a compound
- For example, the Ar of a hydrogen atom is 1, and the Ar of an oxygen atom is 16
- Therefore, a H2O compound, which has 2 hydrogen atoms, and 1 oxygen atoms, has a Mr value of 18 (2 x 1 + 1 x 16 = 18)

- For example, the Ar of a hydrogen atom is 1, and the Ar of an oxygen atom is 16
- A mole is the measurement of an amount of substance
- There are 6.023x10
^{23}particles in a mole- This is called the Avogadro constant

- To calculate the mass of a mole, you have to calculate the Ar or Mr of the element/compound, and add units (grams)
- Example, to calculate a mole of CO2:
- 1 x 12 + 2 x 16 = 44g

- To calculate the number of moles present in a sample, you have to divide the ass by Mr
- Example, to calculate the number of moles in 11g of CO2:
- 11g / 44g = 0.25

- Example, to calculate the number of moles in 11g of CO2:

- Example, to calculate a mole of CO2:

- There are 6.023x10