Quantitative chemistry higher

  • Created by: holly6901
  • Created on: 01-11-18 17:06

The law of conservation of mass

The total mass of reactants = total mass of products

The Law of conservation of mass says no atoms can be created or destroyed in a chemical reaction.

Therefore, the total mass of the reactants must equal the total mass of the products.

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Relative formula (molecular) mass

Mr=sum of Ar

the relative formula mass is equal to the sum of the relative atomic masses of all the atoms in a formula.

To calculate the relative formula mass of a compound, we need to know the relative atomic masses (Ar) of the elements that make the compound. Then we multiply the Ar values by the number of atoms of that element in the compound. Then we add these numbers together.

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Reactions that change mass

  • Burning of wood is a mass decrease because gases such as carbon dioxide and water can escape
  • zinc and hydrochloric acid is a mass decrease as hydrogen is produced and can escape
  • Corrosion of metal is a mass increase because gases from the air add to the metal


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Molecular formula and Empirical formula

The molecular formula shows the actual number of atoms which make up a molecule

eg. Glucose = C6H12O6

The empirical formula shows the simplest ratio of atoms that make up a molecule

eg. glucose = CH2O

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Moles are a measurement of chemical volumes

1 mole of any substance is 6.02 x 10 to the power of 23 atoms of that substance

This is called the Avogadro Constant

The relative formula mass (Mr) of a substance is equal to the mass of 1 mole of that substance (in grams). For an element, this is the same as the relative atomic mass.

Balanced symbol equations can be interpreted in units of moles. This equation shows us that 2 moles of sodium reacts with 1 mole of sulfuric acid to produce 1 mole of sodium sulfate and 1 mole of hydrogen.

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Moles 2

Image result for moles triangle (http://www.bbc.co.uk/staticarchive/f190a2db298b64559185132af90f44924c2d8bf6.png)

If you know the mass and relative formula mass of each substance in a particular reaction, you can work out the moles of each substance and form a balanced equation.

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Limiting reactants

In many chemical reactions, it is important to use up a reactant completely

The reactant that is completely used up in a reaction is called the Limiting reactant because the reaction stops when it is used up.

eg. neutralising acid

It is important no acid remains after the reaction

This is achieved by using an excess (more than necessary) volume of the other reactant

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Percentage Yield

The percentage yield for any reaction is always less than 100%. The following 4 factors make sure the percentage yield of reactions stays below 100%

  • unexpected reactions (side reactions)
  • Waste (chemicals stuck to apparatus)
  • Incomplete reactions (not enough time, reversible so some products turn back into reactants
  • Extraction (difficulty extracting product from the mixture)

The percentage yield of a reaction is calculated as the real yield divided by the theoretical yield multiplied by 100.

theoretical yield - the maximum mass of a product possibly created by a reaction

real yield - the mass of a product created by a reaction

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Atom economy

atom economy(%)=total relative molecular mass of all reactantsrelative molecular mass of desired products×100

The atom economy of a reaction describes the percentage of the reactants that end up as useful products. We can calculate it by dividing the relative molecular mass of desired products by the total relative molecular mass of all reactants, and then multiplying by 100.

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Reaction pathways

Reaction pathways (ways reactions happen) with high atom economies (% of useful products made) are good for 2 reasons:

  • Cost (using the smallest amount of reactants possible saves money)
  • Waste (A higher atom economy means less waste produced)
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The concentration of a solution can be measured in terms of

  • Mass (solution concentration = mass of substance/volume)
  • Moles (solution concentration = moles of substance/volume

We can use information about a solution, to work out which chemicals were in it to start with

We can calculate the amount of a solute (dissolved substance) in a solution if we know the concentration and volume of the solution. 

We can calculate the concentration of a solution if we know the volumes of the solutions that react and the concentration of 1 of the solutions.

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Substance amounts - gases

At the same temperature and pressure, equal amounts of gas (when measured in moles) will take up the same volume.

at room temperature (20 degrees) and pressure (1 atm) 1 mole of any gas takes up 24dm cubed

When given the volume of 1 gas involved in a reaction between gases, the balanced equation for the reaction can be used to calculate the volume of any other component.

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