## The Mole

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A mole is a unit containing approximately 6.02 x 1023 particles. This number, 6.02 x 1023, is called Avagadro's number and is denoted by NA. It is derived from exactly 12 grams of carbon-12. 12 grams of carbon-12 has approximately 6.02 x 1023.
For example, one mole of hydrogen gas (H2) has 6.02 x 1023 molecules of H2.
This unit is important because it forms the basis of writing chemical formulas and equations.
The gram atomic mass is the mass of one mole of particles. For example, the gram atomic mass of carbon is 12.0112, which means that there are 6.02 x 1023 carbon atoms in a sample of 12.0112 grams of carbon at STP.
To find the number of moles of a gas at STP, the volume of the gas must be divided by 22.4 liters. This is because, as Avogadro's law states, equal volumes at the same temperature and pressure contain equal amounts of moles. The volume of 22.4, at STP, is referred to as the mole volume because it contains exactly one mole at STP. For example, at STP, one mole of oxygen gas takes up the volume of 22.4 liters. At STP, a mole of nitrogen gas also takes up the volume of 22.4 liters. Two moles of gas at STP take up the volume of 44.8 liters.
To find the number of moles in another phase, divide the sample mass by its molecular mass. For example 36 grams of water is 2 moles of water because 36 (sample mass) divided by 18 (molecular mass of water) is 2.