A Level Chemistry CCEA

Formulae, Equations and Amounts of Substance

Formulae and Equations

• Understand the concept of chemical formulae used to represent molecules and ions. These convey the type and ratio of atoms in a compound.
• The molecular formula represents the number and type of atoms in a molecule, where the empirical formula conveys the simplest ratio of atoms in a compound.
• Know how to balance chemical equations, which show the number of atoms for each element being conserved in a chemical reaction.
• Reactive species in an equation are termed reactants, while the end products are called products.
• Master the concept of stoichiometry, which is the ratio of reactants to products in a balanced equation.
• Differentiate between molecular and ionic equations, and be able to write net ionic equations by cancelling out spectator ions.

Amounts of Substance

• Familiarize yourself with the term 'mole'. The mole is the unit for amount of substance in chemistry, defined as exactly 6.02 x 10^23 particles.
• Understand the concept of molar mass, expressed in g mol^-1, defined as the mass of one mole of a substance.
• The molar mass of an element can be found on the Periodic Table and is numerically equal to the relative atomic mass.
• Be able to carry out mole calculations using the equation n = m/M where n is the number of moles, m is the mass, and M is the molar mass.
• Understand how to calculate the number of particles in a sample using the number of moles and Avogadro’s constant.
• Know how to calculate molar concentration in mol dm^-3 by using the formula C = n/V where C is the concentration, n is the number of moles and V is the volume in dm^3.
• Familiarize yourself with molar volume at standard temperature and pressure (STP), defined as the volume occupied by one mole of any gas at STP, taken to be 22.4 dm^3 mol^-1.

Limiting Reagents and Excess

• Understand the concept of limiting reagents, the substance that is completely consumed in the reaction and determines the maximum amount of product that can be formed.
• Identify how to determine the limiting reagent in a chemical reaction from the stoichiometry of the equation and mass of reactants.
• Familiarise yourself with calculations involving excess reagents. The reagent that is not completely used up in a reaction is said to be in excess.

Percentage Yield and Atom Economy

• Understand how to calculate percentage yield, a measure of the efficiency of a reaction, calculated with actual yield/theoretical yield x 100%.
• Familiarize yourself with the definition and calculation of atom economy, a concept that assesses the efficiency of a reaction in terms of atom utilization. Atom economy is calculated by (mass of desired product/total mass of reactants) x 100%.