Chemistry 3 - Quantitative Chemistry

Question 1

Relative Formula Mass

Answer 1

Relative formula mass = sum of atomic masses of all atoms

Percentage mass of an element in a compound = Ar * number of atoms of that element / Mr of the compound *100

Question 2

The mole

Answer 2

Avogadros constant = 6.02 * 10^23

Amount of particles where the relative formula mass equals its weight in grams

Number of moles = mass in g ( of an element or compound) / Mr (of the element or compound)

Question 3

Conservation of mass

Answer 3

During a chemical reaction no atoms are destroyed or gained
This means there are the same number and types of atoms on each side of a reaction equation
Because of this no mass is lost or gained - mass is conserved

If the mass changes there is a gas involved either from the products or the reactants as the reaction vessel isn’t closed

Question 4

The mole and equations

Answer 4

1) divide the mass of each substance by its relative formula mass to find the number of moles
2) divide the number of moles of each substance by the smallest number of moles in the reaction
3) if any of the numbers aren’t whole numbers, multiply all the numbers by the same amount so that they all become whole numbers
4) write a balanced symbol equation by putting these numbers in front of the chemical formulas

Question 5

Limiting Reactants

Answer 5

Reactions stop when one reactant is used up
The amount of product depends on the limiting reactant

Calculate the mass of the product formed in a reaction by using the mass of the limiting reactant and the balances symbol equation

1) Write out the balanced equation
2) work out the relative formula masses (Mr) of the reactant and product you want
3) find out how many moles there are of a substance you know the mass of
4) use the balanced equation to find out how many moles there will be of the other substance
5) use the number of moles to calculate the mass

Question 6

Gases and Solutions

Answer 6

One mole of any gas occupies 24dm^3 at 20 degrees C

Volume of gas = (mass of gas / Mr of gas)* 24

Concentration is a measure of how crowded things are

Concentration = mass of solute / volume of solvent

Concentration = number of moles of solute / volume of solvent

Question 7

Concentration Calculations

Answer 7

No of moles = conc. * volume

Converting mol/dm^3 to g/dm^3
Mass = moles * Mr

Question 8

Atom Economy

Answer 8

% of reactants forming useful products

Atom economy = relative formula mass of desired products / relative formula mass of all reactants * 100

High atom economy is better for profits and the environment

Question 9

Percentage Yield

Answer 9

Percentage yield compares actual and theoretical yield

Percentage yield = mass of product actually made / maximum theoretical mass of product * 100

Yields are always less than 100% - in real life

• reversible reaction the product can turn back
• may be side reactions ( with gases in the air)
• you lose some product when you separate it from the reaction mixture