Moles

Definition• A mole is the mass of a substance

which contains the same number of particles as 12 grams of the isotope carbon12.

• These particles could be atoms, ions or molecules.

• 12 grams of carbon12 contain 6.02 x1023

atoms• This number is the Avogadro Constant,

and is given the symbol L.

Eg; One mole is…

27g of aluminium63.5g of copper

16 + 2 = 18g of water

Calculating the number of particles

• Number of particles = moles x Avogadro Constant.

• Eg; How many particles are there in 0.1 moles of hydrogen?

• N = 0.1 x 6.02x1023 = 6.02 x 1022

molecules.

Calculating the number of moles

• Moles = number of particles / Avogadro constant.

• Eg; How many moles of methane are there in 3.01 x1023 molecules?

• Moles = 3.01x1023 / 6.02x1023 = 0.5 moles.

Calculating the molar mass

• One mole of an element has a mass equal to the relative atomic mass in grams.

• Eg; the mass of one mole of sodium is 23g.

• The mass of one mole of a compound is found by adding together the masses of all its elements in the correct ratio.

• Units gmol-1

• Eg; What is the molar mass of calcium carbonate?

• Write the formula; CaC03.

• M = RAMCa + RAMC + 3xRAMO.• M = 40 + 12 + 3x16 = 100g

Calculating the number of moles

• No moles = mass / RAM• Eg; How many moles are there in

4g of calcium.• Look up the mass number of

calcium in the periodic table.• The atomic mass is the mass

number in grams. • N = 4 / 40 = 0.1 moles

Number of moles

Mass (g)

Relative atomic mass

n

m

RAM

Think of the equation as a triangle

Number of moles =

mass (g)

Relative atomic mass

n

m

RAM

n = m / RAM

Rearranging;

Calculating the number of moles of a compound.

• No moles = mass / Mr• Eg; How many moles are there in 15g of

calcium carbonate?• Look up the Ars, then add them

together to find the Mr.• Mr = 40 + 12 + (16 x 3) = 100• One mole has a mass of 100g.• N = 15 / 100 = 0.15 moles.

Number of moles

mass =

n

m

RAM

m = nRAM

X RAM

Rearranging;

Calculating the mass from moles• Mass = No moles X Ar• Eg; What is the mass of 0.75 moles of

sodium?• Ar = 23• Mass = 0.75 x 23 = 17.25g• Similarly for a compound;• Mass = No moles X Mr• Eg; What is the mass of 0.8 moles of

sodium chloride?• Mr = 23 + 35.5 = 58.5• Mass = 0.8 x 58.5 = 46.8g