GCSE AQA Chemistry C3

What is the law of conservation of mass?

Mass is never created or destroyed during a chemical reaction

How does the total mass of reactants compare to the total mass of products in a chemical reaction?

The total mass of products equals the total mass of reactants

What is the relative formula mass (Mr) of a compound?

The sum of the relative atomic masses of all the atoms in the formula of a compound

Does relative formula mass (Mr) have units?

No

Why might a reaction carried out in an open beaker appear to gain mass?

A gaseous reactant from the air has reacted and bonded with the solid or liquid reactants

Why might a reaction carried out in an open beaker appear to lose mass?

A gaseous product has been produced and escaped into the surrounding air

What is chemical uncertainty?

The range of measurements within which the true value of a quantity is expected to lie

How do you calculate uncertainty from a set of repeat measurements?

Uncertainty = Range / 2

What is a mole?

The unit for the amount of a chemical substance

What is the value of the Avogadro constant?

6.02 x 10^23 per mole

What does the Avogadro constant represent?

The number of particles, atoms, molecules or ions in one mole of any substance

How much does one mole of any substance weigh in grams?

A mass equal to its relative formula mass (Mr) or relative atomic mass (Ar)

State the equation linking mass, moles and relative formula mass

Mass = Moles x Mr

State the equation to calculate the number of particles in a given amount of substance

Number of particles = Moles x Avogadro constant

What do the large numbers in front of chemical formulas in a balanced equation represent?

The relative number of moles of each reactant and product

How can you balance an equation using the masses of reactants and products?

Convert the mass of each substance to moles and simplify the mole ratio to whole numbers

What is a limiting reactant?

The reactant that is completely consumed first in a chemical reaction, limiting the amount of product formed

What is an excess reactant?

A reactant that is present in a quantity greater than necessary to react completely with the limiting reactant

Why is one reactant often added in excess in a chemical reaction?

To ensure that the limiting reactant is completely used up

What is concentration?

A measure of the mass or number of moles of a solute dissolved in a given volume of solvent

How many cubic centimetres (cm3) are in one cubic decimetre (dm3)?

1000 cm3

State the equation linking concentration in g/dm3, mass in g, and volume in dm3

Concentration (g/dm3) = Mass (g) / Volume (dm3)

State the equation linking concentration in mol/dm3, moles, and volume in dm3

Concentration (mol/dm3) = Moles / Volume (dm3)

How do you convert a volume from cm3 to dm3?

Divide by 1000

How do you convert a concentration from mol/dm3 to g/dm3?

Multiply by the relative formula mass (Mr) of the solute

What is the theoretical yield of a reaction?

The maximum theoretical mass of product that could be formed from a given amount of reactants

What is the actual yield of a reaction?

The mass of product that is actually obtained when a reaction is carried out in practice

State the equation to calculate percentage yield

Percentage yield = (Actual yield / Theoretical yield) x 100

Give three reasons why a percentage yield is rarely 100%

The reaction may be reversible, some product may be lost during separation/filtration, or unexpected side reactions may occur

What is atom economy?

A measure of the proportion of starting materials that end up as useful products

State the equation to calculate percentage atom economy

Percentage atom economy = (Mr of desired product / Sum of Mr of all reactants) x 100

Why is a high atom economy important in industrial processes?

It minimises waste, saves money on raw materials, and conserves resources

What is the atom economy of a reaction that produces only one single product?

100%

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

24 dm3 (or 24,000 cm3)

What are room temperature and pressure (RTP)?

20 degrees Celsius and 1 atmosphere of pressure

State the equation linking volume of gas in dm3, moles, and molar volume

Volume of gas (dm3) = Moles x 24

State the equation linking volume of gas in cm3, moles, and molar volume

Volume of gas (cm3) = Moles x 24000

What is a titration used for?

To accurately determine the concentration of an acid or an alkali by neutralisation

Which piece of apparatus is used to accurately measure a fixed volume of alkali or acid into a conical flask?

A volumetric pipette

Which piece of apparatus is used to add acid or alkali dropwise until neutralisation occurs?

A burette

Why is a conical flask used instead of a beaker in a titration?

Its sloped sides prevent splashing when swirling the mixture

Why is a single indicator like phenolphthalein used instead of universal indicator in a titration?

It gives a sharp and distinct colour change at the endpoint

What colour change occurs when phenolphthalein is added to an alkali and neutralised with acid?

From pink to colourless

What are concordant results in a titration?

Titre volumes that are within 0.10 cm3 of each other

How do you calculate the mean titre from a set of titration results?

Calculate the mean of only the concordant results, ignoring the rough trial and non-concordant outliers