Core Chemistry Concepts: Moles, Isotopes, Concentration & Yield

Conservation of Mass

Conservation of mass: No atoms are lost or created during a chemical reaction. The mass of reactants equals the mass of products. Any observed change in mass typically involves a gas being produced or lost as a result of the reaction.

Isotopes and Relative Formula Mass (RFM)

Isotopes and RFM:

RAM — Relative atomic mass. RFM or Mr — relative formula mass; the sum of all RAMs in a formula.

Percentage abundance calculation (for an isotope mixture):

% abundance = (RAM × number of atoms of that element / RFM of the compound) × 100.

An isotope is an atom of an element that has the same number of protons and electrons but a different number of neutrons, so it has a different mass number. Isotopes of the same element react in the same way chemically because they have the same electron configuration.

Relative atomic mass from isotopic abundances:

(mass of 1st isotope × % abundance) + (mass of 2nd isotope × % abundance) / 100

Moles and Chemical Equations

Moles: moles = mass (g) / RFM. The RFM (or Mr) of an element or compound is the mass in grams that corresponds to 1 mol.

Avogadro's number: 1 mole of a substance = 6.022 × 10²³ particles (Avogadro's number).

Working out expected mass of a substance from an equation:

1. Work out what you know.
2. Work out the mole ratio from the balanced equation.
3. Work out the mass of the unknown.

Limiting reactant: A limiting reactant is the reactant in a reaction that has fewer moles available, so it limits the amount of product formed. For example: if you have 1 mol of HCl and 2 mol of NaCl (note: this is an illustrative example), HCl would be the limiting reactant and NaCl would be in excess.

Concentration and Gas Volume

Concentration (g/dm³): concentration = mass (g) / volume (dm³).

Gas volume: Number of moles in a gas = volume (dm³) / molar gas volume.

At typical room conditions used here, the molar gas volume is taken as 24 dm³ per mol. Therefore:

moles = volume (dm³) / 24

Concentration (for solutions): concentration = moles / volume (dm³).

Atom Economy

Atom economy: Atom economy measures the proportion of the starting materials that end up as the desired product. By minimising the production of unwanted products, we save money and increase sustainability by reducing waste of resources. An efficient reaction is one that produces little or no waste products. Chemists often try to find uses for side products (for example, using them in other reactions) to reduce waste.

Atom economy (%): (RFM of desired product(s) / RFM of all reactants) × 100.

Atom economy cannot exceed 100% because that would imply atoms are being created.

Yield

Yield: It is not always possible to achieve 100% yield in a chemical reaction. There are three common reasons for this:

1. Some of the product may be lost during separation from the reaction mixture.
2. Some reactants may undergo side reactions and not form the expected product.
3. Reversible reactions may not go to completion.

Yield (%) = (mass of actual product / maximum theoretical yield) × 100.

Yield cannot be more than 100%. Higher purity of product generally corresponds to a more reliable (effective) yield measurement.