mole concept and stoichiometry

What is relative atomic mass(Mr)

Weighted average mass of molecules of an element or compound when compared with the mass of 1/12 of an atom of the carbon-12 isotope

What is relative atomic mass (Ar)

Weighted average mass of atom of an element when compared with the mass of 1/12 of an atom of the Carbon-12 isotope (Ar can be found in the periodic table number aat the bottom)

How to calculate Mr

Add up all the Ar of all the atoms present in the chemical formula

Why is carbon-12 isotope chosen as standard for comparison

• cheap and easily available

• Solid at room temperature, easy to measure mass

• 12 times the mass of hydrogen atom hence 1/12 of carbon atom has the same mass as 1 hydrogen atom

• Use mass of 1/12 carbon-12 isotope in place of 1 hydrogen atom to compare mass of atoms

Why Ar not whole number

Because most naturally occurring elements contain a mixture of isotopes in different proportions

Ar is taken by taking weighted average mass of the isotopes in different proportions

What is relative formula mass

• Describe ionic compounds because they do not exist as molecules

• Weighted average of a formula unit of the ionic compound when compared with the mass of 1/12 of an atom of the carbon-12 isotope

What does mole mean Aka avagadros constant

One mole of a substance will always contain 6.02×10²³ particles

Unit is mol

What is molar mass

Mass of one moles of a substance

Numerically equal to Ar or Mr

Unit: g/mol

Formula for calculating number of moles

Number of moles= mass(g)/ molar mass

How to calculate % by mass of element

%by mass of element + sum of element/ Mr of compound

How to calculate % by mass of water of crystallisation

% by mass of water if crystallisation= sum of Mr of water/ Mr of compound

What is sstoichiometry

Ratio between the quantities of substances involved in a chemical reaction

Ratio between the different quarantines of different substances is the mole ratio

Must mass of reactants = mass of products assuming there is no excess reactants

Stoichiometry based on the fact that matter cannot be destroyed or created in a chemical reaction

Known as the Law of Conservation of Mass Where total mass of all reactants must be equal to the total mass of all the products

What is empirical formula

Simplest whole number ratio of the atoms of each element present in a compound

How to calculate empirical formula

1. Calculate mass assuming 100g if given %, masses = %mass

2. No of moles (/by Mr)

3. Divide by smallest number of moles round to 1dp

4. Mole ratio

How to process mole ratios

Multiply by common integer to get whole number ratio of

• convert 1:1.5 to 2:3 (multiply by 2 throughout)

• Convert 1: 1.33 to 3:4 (multiply by 3 throughout)

What is molecular formula

Shows the actual number of atoms of each element found in one molecule of the compound (same meaning as chemical formula but it only refers to covalent substances)

How to determine the molecular formulas

n= Mr of compound/ Mr of empirical formula

What is molecular volume

• Molar volume of gas is the volume occupied by 1 mole of the gas

• At room temperature and 1 atmosphere, one mole of any gas has volume of 24dm³ (1dm³=1000cm³)

How to calculate number of moles of a gas (room temp and pressure)

Number of moles= gas volume(dm³)/ 24dm³

What is Avogadro’s law

Equal volumes of gases, at the same temperature and pressure, contain equal number of molecules

Thus equal volumes of gases contain equal number of moles

Volume ratio= mole ratio for gases at the same temperature and pressure (apply to chemical formula)

Conditions for using volume ratios of gases

• same temp and pressure

• Gas compared to gas (there could be other states in the reaction

• Used only for gas volumes, not masses

How to calculate % purity

%purity = mass of pure reactant (g)/ mass of impure sample (g) * 100%

Assumptions made when calculations % purity

• impurities in the impure sample do not react with the acid/alkali added

• The products formed during the reaction are pure

In practice wha are possible reasons why %yield of products is n or 100%

• reaction is reversible

• Side reactions

• Some substances may. Be lost due to evaporation, spillage, left in the dilution and not crystal listed

• The reactants are not pure : (the more impure the reactants, the lower the actual yield of products)

What is theoretical expected mas of products

• The mass of product that is the expected mass to form when all the given reactant reacts

• It is the maximum possible mass of products calculated using mole ratios

What is the experimental mass of product

• Actual mass of product obtained in an experiment

• The actual mass varies from experiment to experiment ( value is given in question)

How to calculate % yield

% yield = experimental mass (g)/ theoretical (g) * 100%

What is a saturated solution

Solution that contains the maximum mass of solute dissolved in a fixed volume of solvent at a particular temperature

Additional solute added will not dissolve

How to calculate concentration of solution in g/dm³

Cencentration (g/dm³) = mass(g)/ volume (dm³)

How to calculate concentrations of solution in mol/dm³

Concentration (mol/dm³) = number of moles/ volume (dm³)

Relationship between concentration of solution in g/dm³ and concentration in mol/dm³

Concentration (mol/dm³) = concentration (g/dm³) / molar mass

What is excess reactant

• the substances that is not used up completely during a reaction the amount is more than enough to react

What is limiting reactant

• The reactant that is completely used ip in a reaction

Impacts of limiting reactants

• limits reaction

• number of moles of limiting reatant determines the number of moles the product formed since it stops the reaction before all the excess has reacted

Why is it important to identify limiting reactant

• to get maximum yield at minimum cost , cheaper is used in excess and expensive is limiting reactant to ensure all of it is used up

• Identify excess and limiting reactant and use the limiting reactant in the question

How to calculate excess and limiting reactants

1. Write balanced chemical equation

2. Calculate the moles of both reactants available

3. Calculate moles of reactant B needed if reactant A is fully reacted

4. Compare the moles of reactant B needed and the moles of reactant B available