# Calculations, Moles and Yield

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## The Mole and Conservation of Mass

The Mole

• amount of a substance
• Avogadro's Constant 6x10(23)- how many particles
• mass of atoms/molecules = same as relative atomic mass
• number of moles= mass in g (of element)/mr (of element)

Conservation of Mass

• no atoms created/destroyed
• same number/type of atom on each side
• total mr of recatants=total mr of products
• if mass changes it is because a gas (which was in the air), helps form the product making it heavier
• if mass=smaller gas has escaped in reaction
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## Limiting Reactants

Reactions stop when a reactant is used up.

• anything added in excess is to make sure the other reactant is used up
• reactant thats used up=limiting reactant
• half limiting reactant=half product- vice versa
• product formed=directly proportional to amount of limiting reactant
• to calculate mass of product:
• balanced equation
• work out Mr
• (moles=mass/mr mass/mr)
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## Atom Economy

Atom economy=%of useful products

• atom economy= mr of desired products/mr of reactants x100
• 100% atom economy means all atoms in reactants turned into useful products
• Problems:
• low economy=onsume a lot of resources
• waste (need to be disposed of) -expensive to do
• not profitable
• need to use waste products not throw away
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## Percentage Yield

Percentage yield compare actual+theoretical

• %yield=mass of actual product/max theoretical mass product x100
• high yield reduces waste and cost
• yields are always less than 100%
• not all reactants make a product (reversible reactions, can turn back
• possible side reactions= extra product (react with air)
• lose product when seperating from reaction mixture
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