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Moles

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Moles

- 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…

16 + 2= 18g ofwater

63.5g of copper

27g ofaluminium

- Number of particles = moles x Avogadro Constant.
- Eg; How many particles are there in 0.1moles of hydrogen?
- N = 0.1 x 6.02x1023 = 6.02 x 1022molecules.

- 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.

- 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

- 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

Think of the equation as a triangle

Mass (g)

m

Relative atomic mass

RAM

n

Number of moles

Rearranging;

n = m / RAM

mass (g)

m

Number of moles =

Relative atomic mass

RAM

n

- 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.

Rearranging;

m = nRAM

mass =

m

n

RAM

Number of moles

X

RAM

- 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