Chemistry 5 - Quantative Chemistry (AS)
- Created by: Alexander Hyde
- Created on: 27-03-22 11:01
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
3. Multiply by Avogadro's constant
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
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
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.
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)
Reactions
Displacement reactions
A more reactive element replaces a less reactive element in a compound. e.g.
Cl2 + 2KBr ---> Br2 + 2KCl2
Reactions of acids
When a base reacts with an acid, a salt and water is produced. e.g.
H2SO4 + 2NaOH ---> Na2So4 + 2H2O
Precipitation reactions
Reaction of two aqueous compounds to form a solid.
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)
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)
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
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%
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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