# Chemistry 5 - Quantative Chemistry (AS)

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## The Mole

Number of atoms

The mole is just a name for the number of particles you have.

1 mole = 6.02x1023 particles - This number is known as Avogadro's constant

To work out the number of particles in a substance:

1. calculate the molar mass

2. Calculate the number of moles

4. If needed, multiply by the number of atoms in the molecule. E.g. H2S has 3 particles so multiply by 3.

Molar Mass

The molar mass is exactly the same as Mr and tells you the mass per mole of something. For example, the molar mass of carbon (C) = 12.0g mol-1

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## Calculations involving moles

Number of moles

Number of moles = Number of particles/Avogadro's constant

Moles in a solid

n = m/Mr

n = moles (mol), m = mass (kg), Mr = molar mass (g/mol)

Moles in a solution

n = c*v

n = moles (mol), c = concentration (g/cm^3), v = volume (dm^3)

moles in a gas

Number of moles = volume/Molar gas volume

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## Empirical and Molecular Formulae

Empirical formulae

The Empirical formulae gives you the simplest whole number ratio of atoms in a formula.

To calculate the empirical formulae:

1. Write down the elements involved

2. write down the masses (or percentages as masses)

3. Divide these by the relative atomic masses

4. Divide by the smallest number of moles

Molecular formulae

The molecular formulae gives you the actual ratio of atoms in a formula.

Given the empirical formulae, to calculate the molecular formula of the reaction, work out the Mr of the empirical formula and divide by the Mr of the molecular formula. Use that result to multiply by the number of atoms in the empirical formula.

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## Equations

Balanced equations

Balanced equations have equal numbers of each atom on each side. To get this, you can put multipliers in front of a chemical.

Ionic equations

Ionic equations show you the ions involved in a reaction. Only reacting substances are included.

Given a balanced equation of a reaction, Write out all the ions in the equation and then remove ions which appear on both sides.

Reaction Stoichiometry

You can use the ratio to calculate the amount of a substance produced elsewhere in a reaction.

State symbols

s = solid, l = liquid, g = gas, aq = aqueous (solution in water)

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## Reactions

Displacement reactions

A more reactive element replaces a less reactive element in a compound. e.g.

Cl+ 2KBr ---> Br2 + 2KCl2

Reactions of acids

When a base reacts with an acid, a salt and water is produced. e.g.

H2SO4 + 2NaOH ---> Na2So+ 2H2O

Precipitation reactions

Reaction of two aqueous compounds to form a solid.

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## Gas calculations

The ideal gas equation

pV = nRT

p = pressure (Pa)

V = Volume (m3)

n = Moles (Mol)

R = 8.31 J K-1 mol-1

T = Temperature (K)

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## Titration calculations

Concentrations

1. Write a balanced equation and write out what you know

2. Work out how many moles of acid you have.

3. Use the molar ratio to calculate how many moles of solution you have

4. Calculate the final concentration of the solution (C = m/v)

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## Uncertainty

Uncertainty arises because you cannot be 100% accurate in your measurements. This may simply be due to limitations in the equipment.

Percentage uncertainty

%age uncertainty = Uncertainty/reading * 100

Total uncertainty

1. Find the percentage uncertainties for each bit of equipment

2. Add them together to give the final uncertainty in the result

3. Calculate the actual uncertainty

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## Atom economy

Atom economy is the measure of the efficiency of the reaction.

% atom economy = Mr of desired product/Mr of all products *100

In addition reactions, the atom economy is always 100%

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## Percentage yield

The percentage yield is the maximum product you can get if no chemicals are 'lost' in the process.

Percentage yield = actual yield/theoretical yield * 100

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