Calculations

Calculations

C2 Revision

Ar and Mr

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

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)

Percentage Composition by Mass 

Percentage composition by mass  =

Ar of element in the compound

Mr of the compound

× 100

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

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

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  

Balancing Equations

•Law of Conservation:

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

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

 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

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

Percentage Yield continued...  

Percentage

 yield =

     Actual Yield g

Theoretical Yield

× 100

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

Atom Economy continued...

Atom

Economy =

Mr of all used products

Mr of all Products

× 100