# Calculations

i hope these help!

• Created by: Christina
• Created on: 15-01-11 14:41

Calculations

C2 Revision

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Ar and Mr

The Relative Atomic Mass à Ar The Relative Formula Mass à Mr

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Working Out the Mr

Step 1 – Find the Ar’s of all elements in the compound Step 2 – Add them together (if there is more than one element, for example 2, then multiply the Ar by however many number of elements there are)

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Percentage Composition by Mass

Percentage composition by mass  =

Ar of element in the compound

Mr of the compound

× 100

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Formulas

A formula has to have a whole number in it. Two types: àEmpirical – in its simplest form        Two types: àe.g. HO  Two types: àMolecular – not in its simplest  Two types:   form    wo types: àe.g. C₂H₆

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Calculating Empirical Formulas –
Percentages and Masses

Step 1 – Divide each mass by the Ar of its element Step 2 – Divide each of the answers by the smallest answer Step 3 – Use the ratio to give a whole number formula This works with both percentages and masses

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Calculating Molecular Formulas – Percentages and Masses

Step 1 – Divide each mass by the Ar of its element Step 2 – Divide each of the answers by the smallest answer Step 3 – Find the Empirical Formula Step 4 – Work out its Mr Step 5 – See *** many times the Mr goes into the given molecular mass Step 6 – Multiply the empirical formula by the previous answer This works with both percentages and masses

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Balancing Equations

Law of Conservation:

“No atoms of an object can be either created or destroyed”

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Mole Calculations 1 Mole = 610²³ The Ar of any element in grams contains 1 mole of atoms Remember: Mass = Moles × Ar/Mr Moles = Mass ÷ Ar/Mr Ar/Mr = Mass ÷ Moles

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Calculating the Mass of an element/compound from a given mass

Step 1 – Write a balanced symbol equation for the reaction (You will always be given this)

Step 2 – Identify the ‘known’ and the ‘unknown’

Step 3 – Calculate the number of moles of the ‘known’ value Step 4 – Find the link (u÷k) Step 5 – Find the number of moles of the ‘unknown’ value Step 6 – Calculate the mass of the ‘unknown’ value

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Percentage Yield

Theoretical Yield is the amount you should get Actual Yield is the amount you do get You get the theoretical yield from a calculator You get the actual yield from an experiment You will always be given both the theoretical yield and the actual yield

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Percentage Yield continued...

Percentage

yield =

Actual Yield g

Theoretical Yield

× 100

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Atom Economy

Atom Economy is all about trying to find out how efficient a reaction is Example: 2H₂ + O₂ à 2H₂0 is a synthesis reaction because it only makes one product NaOH + HCl à NaCl + H₂O The first reaction is more efficient because it has no waster products and you get more H₂O than the second reaction

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Atom Economy continued...

Atom

Economy =

Mr of all used products