Quantative chemistry C3

Conservation of Mass

• No atoms are destroyed and no atoms are created during a chemical reaction. • The total mass of the reactants always equals the total mass of the products.

Mass drop in open systems

• If mass appears to decrease, it is because a product is a gas. • The gas escapes from the unsealed reaction vessel into the air, so its mass cannot be weighed.

Mass rise in open systems

• If mass appears to increase, it is because a reactant is a gas. • Gas from the surrounding air enters the vessel and reacts to form part of the solid product.

Relative Formula Mass (Mr)

• The sum of the relative atomic masses (Ar) of the atoms in the numbers shown in the chemical formula. • It has no units.

Mr balanced equation rule

• In a balanced chemical equation, the sum of the total Mr of the reactants equals the sum of the total Mr of the products.

The Mole definition

• The unit for the chemical amount of a substance, symbolised as mol. • The mass of one mole of a substance in grams is numerically equal to its relative formula mass (Mr).

Avogadro Constant

• The number of atoms, molecules, or ions in a mole of any given substance. • Value is 6.02 x 10²³ per mole.

Moles formula

• Number of Moles = Mass (g) ÷ Relative Formula Mass (Mr).

Do not include the big balancing number when calculating the Mr.

Limiting Reactant definition

• The reactant that is completely used up first in a chemical reaction. • It restricts and limits the maximum amount of product that can be formed.

Limiting Reactant product link

• The amount of product formed is directly proportional to the amount of the limiting reactant used.

Excess Reactant definition

• The reactant that is not completely used up during a chemical reaction.

• It is added in a larger amount to ensure that the limiting reactant reacts completely.

Concentration formula (Mass)

• Concentration (g/dm³) = Mass of solute (g) ÷ Volume of solvent (dm³).

Volume unit conversion

• To convert volume from cubic centimetres (cm³) to cubic decimetres (dm³), you must divide by 1000.

Concentration formula (Moles)

• Concentration (mol/dm³) = Number of Moles ÷ Volume of solvent (dm³).

Percentage Yield definition

• A measure of the chemical efficiency of a practical reaction. ( we can change and it can never be greater than 100%).

Percentage Yield formula - Triple Only

• Percentage Yield = (Actual Mass obtained ÷ Maximum Theoretical Mass) x 100.

Why Yield is never 100%

The chemical reaction may be reversible and therefore does not go to full completion.

Some of the product may be lost mechanically when it is separated or filtered from the reaction mixture.

Some of the reactants may react in unexpected, side reactions instead of the main reaction.

Atom Economy definition

The percentage from the mass of reactants that becomes the desired, useful product and not waste- the efficiency of the natural chemical reaction (we cannot change)

Atom Economy sustainability

• High atom economy is important for sustainable development to preserve resources, environmentally better (less toxic/harmful products) and reduce economic waste costs.

Atom Economy formula

• Atom Economy = (Mr of Desired Product ÷ Sum of Mr of All Reactants) x 100.

You must include the big balancing numbers from chemical equation when calculating Mr

Molar Gas Volume rule

• Equal amounts of moles of gases occupy equal volumes under the same conditions of temperature and pressure.

One Mole of Gas volume constant - Triple Only

• One mole of any gas occupies a volume of exactly 24 dm³ at room temperature and pressure (RTP).

Gas Volume formula - Triple Only

• Volume of Gas (dm³) = Number of Moles x 24.