Unit 3 - Quantitative Chemistry


Relative Formula Mass

The Relative Formula Mass of a compound is the relative atomic masses of all the atoms in the molecular formula added together.

For example MgCl :

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One mole of any substance is just the amount of that substance that contains an Avogardo number of particles which is 6.02 x 10  particles. This could be atoms, molecules, ions or electroons. This is because the mass of that number of atoms or molecules of any substance is exactly the same number of grams as the relative atomic mass or relative formula mass of the element or compound.

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Mole Equations

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Limiting Reactants

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Gases and Solutions

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Concentration Calculations

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Atom Economy

Atom Economy is the percentage of reactants forming useful products. A lot of reactions make more than one product. Some of them will be useful but othhrs will be a waste. 

High Atom Economy is better for profits and the environment:

  • Reactions with low Atom Economy use up resources very quickly. At the same time, they make lots of waste iaterials which have to be diposed of somehow. This tends to make these reactions unsustainable  as the raw materials run out and the waste has to go somewhere.
  • For the same reasons, low Atom Economy reactions aren't usually profitable becaust raw materials are often expensive to buy and waste products can be expensive to remove and dipose of properly. 
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Percentage Yield

The amount of product you get is known as the yield. Percentage Yield compares Actual and Theoretical Yield. Yields are always less than 100%:

  • Not all reactants react to make a product - in resversible reactions the products can turn back into reactants therfore the yield will never be 100%.
  • There might be side reactions - the reactants sometimes react differently to how you expect. They might react with the gases in the air or impurities in the recation mixture, so they end up forming extra products other then the ones you want.
  • You lose some product when you seperate it from the reaction mixture.


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Conservation of Mass

During a chemical reaction, no atoms are oestroyed and no atoms are created. This means there are the same number and types of atoms on each side of the equation. Becausc of this, no mass is lost or gained so we say that mass is conserved during a reaction. By adding up all of the relative formula masses of the substances on each side of the equation you can see that mass is conserved.

In some experiments, you might observe a change in mass of an usealed reaction vessel during a reaction. There are usually two explanations for this:

  • If the mass increases, it's probably because one of the reactants is a gas that is found in air e.g oxygen and all the products are solids, liquids or aqueous.
  • If the mass decreases, it's probably because one of the products is a gas and all the reactants are solids, liquids or aqueous.
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