GCSE AQA Chemistry - Quantitative Chemistry

Relative atomic mass

The average mass of the isotopes of the element, taking into account the abundance of each isotope.

Relative formula mass

The sum of the relative atomic masses in a formula.

Percentage mass of an element in a compound =

(A_r x number of atoms of that element / M_r of the compound) x 100

Avogadro’s number (1 mole =)

6.02×10²³

moles (of an element) =

mass (g) / relative atomic mass A_r

moles (of an compound) =

mass (g) / relative formula mass M_r

Conservation of mass

During a chemical reaction no atoms are destroyed or created.

Why may mass appear to increase in a chemical reaction?

• Because one of the reactants is a gas and all of the products are not gases

• The mass of the gas cannot be accounted for if it is not contained in the reaction vessel

Why may mass appear to decrease in a chemical reaction?

• Because one of the products is a gas and all of the reactants are not gases

• The mass of the gas cannot be accounted for if it is not contained in the reaction vessel

What do the big numbers in front of substances in a reaction equation tell us?

The moles of each substance required for them to react.

Limiting reactant

The reactant that is completely used up in a reaction, so limits the amount of the product that can be formed.

What volume does one mole of any gas occupy at room temperature and pressure?

24dm³

Room temperature

20°C

Room pressue

1 atmosphere

Volume of gas (dm³) =

(Mass of gas (g) / M_r of gas) x 24

Concentration

The amount of a substance in a certain volume of a solution.

Concentration (g/dm³) =

Mass of solute (g) / volume of solvent (dm³)

1dm³ =

1000cm³

Concentration (mol/dm³) =

number of moles of solute (mol) / volume of solvent (dm³)

Atom economy

The percentage of mass of products that are useful.

Atom economy =

(M_r useful product / M_r all reactants) x 100

How is high atom economy more profitable?

• Less money spent on disposing of waste products

• Less money spent on reactants to get the same mass of product

How is low atom economy unsustainable?

• Uses lots of resources quickly

• Produces lots of waste materials that must be disposed of

Yield

The amount of product obtained.

Percentage yield

The actual yield as a percentage of the maximum theoretical amount.

Percentage yield =

(Mass of product actually made / maximum theoretical mass of product) x 100

What are the reasons for yield never being 100%?

• The reaction may be reversible so will not go to completion

• There may be side reactions

• Some product may be lost when separated from the reaction mixture