Unit 2 Calculations

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Relative Formula Mass

Relative formula mass (Mr) = the atomic masses added together

Realtive formula mass of a substance of grams is known as one mole of that substance

NUMBER OF MOLES = Mass in g


Mr

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Percentage Mass of an Element in a Compound

Percentage mass = Ar x no.of atoms (of that element)
-------------------------------------------- x100
Mr

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Emperical Formula

1) List all the elements in the compund
2) Underneath them, write their experimental masses or percentages
3) Divide each mass or percentage by the Ar for that particular element
4) Turn numbers you get into a nice simple ratio
5) Leave ratio in it's simplest formula

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Calculating Masses

1) Write out the balanced equation
2) Work out Mr - just for the two bits you want
3) Apply rule: Divide to get one, then multiply to get all (You have to apply this first two the substance you are given the info for, they apply to the other)

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Percentage Yield

Compares actual and predicted yeild

Percentage yeild = actual yield (g)
------------------------- x100
predicted yield (g)

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Relative Formula Mass Example

Q: How many moles are there in 42g of carbon
A: No. of moles = Mass (g) Mr = 4212 = 3.5 moles

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Percentage Mass of an Element in a Compound Exampl

Q: Find the percentage mass of sodium in sodium carbonate, Na2CO3
A: Ar of sodium = 23, Ar of carbon = 12, Ar of oxygen = 16
Mr = 2x23 + 12 + 3x16 = 106

23x2
------- x100 = 43.4%
106

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Emperical Formula Example

Q: Find the emperical formula of the iron oxide produced when 44.8g of iron react with 19.2 g of oxygen

A: 1) List the 2 elements: Fe O
2) Write experimental masses: 44.8 19.2
3) Divide by Ar of each element: 44.8/56 = 0.8 19.2/16 = 1.2
4) Multiply by 10: 8 12
5) Divide by 4: 2 3

Fe2O3

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