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neurologyYou need to know

• The law of conservation of mass states that atoms are not created or destroyed in a chemical reaction, so the total mass of reactants equals the total mass of products in a closed system.
• A balanced symbol equation has the same number of each type of atom on both sides of the equation because atoms are rearranged, not lost or made.
• In an open container, a reaction may appear to lose mass if a gaseous product escapes, or appear to gain mass if a gaseous reactant from the air is used.
• Chemical formulae show the elements present in a substance and the number of atoms or ions of each element in one formula unit.
• State symbols identify physical states in equations: solid is `(s)`, liquid is `(l)`, gas is `(g)`, and aqueous solution is `(aq)`.
• A mole is an amount of substance containing `6.02 × 10^23` particles, called the Avogadro constant.
• The particles counted by a mole can be atoms, molecules, ions, electrons, or formula units, depending on the substance.
• The relative atomic mass, `A_r`, is the weighted mean mass of an atom of an element compared with one twelfth of the mass of a carbon-12 atom.
• The relative formula mass, `M_r`, is the sum of the relative atomic masses of all the atoms shown in a chemical formula.
• The molar mass of an element or compound is numerically equal to its `A_r` or `M_r`, but it has the unit grams per mole.
• The number of moles is calculated using `moles = mass ÷ molar mass`.
• The mass of a substance is calculated using `mass = moles × molar mass`.
• Percentage abundance means the proportion of each isotope in a sample of an element, expressed as a percentage.
• The relative atomic mass of an element with isotopes is found from the sum of each isotope mass multiplied by its fractional or percentage abundance.
• The balancing numbers in a chemical equation give the mole ratio of reactants and products in the reaction.
• For example, `Mg + 2HCl → MgCl₂ + H₂` means one mole of magnesium reacts with two moles of hydrochloric acid to form one mole of magnesium chloride and one mole of hydrogen.
• The limiting reactant is the reactant that is completely used up first and therefore determines the maximum amount of product that can form.
• A reactant in excess is present in a greater amount than needed, so some of it remains unreacted when the limiting reactant is used up.
• Concentration in grams per cubic decimetre is the mass of solute dissolved in each cubic decimetre of solution, so `concentration = mass ÷ volume`.
• At room temperature and pressure, one mole of any gas occupies a volume of about `24 dm³`.

• Balance a chemical equation by changing coefficients only, checking that the number of atoms of each element is equal on both sides.
• Calculate an unknown mass in a reaction using conservation of mass by adding the known reactant masses and product masses and finding the missing value.
• Calculate relative formula mass by multiplying each element’s `A_r` by the number of its atoms in the formula and adding the totals.
• Calculate relative atomic mass from isotope data by multiplying each isotope mass by its percentage abundance, adding the results, and dividing by 100.
• Convert between mass, moles, and molar mass using `moles = mass ÷ molar mass` and `mass = moles × molar mass`, with mass in grams.
• Use a balanced equation to calculate an unknown mass by converting the known mass to moles, applying the mole ratio, and converting the answer back to grams.
• Identify the limiting reactant by comparing the available moles of reactants with the mole ratio required by the balanced equation.
• Calculate concentration, mass, or volume of a solution using the relationship between mass of solute, volume of solution, and concentration in `g/dm³`.
• Calculate an unknown concentration from a neutralisation reaction by using the balanced equation mole ratio and the measured volumes of acid and alkali.
• Calculate gas volumes at room temperature and pressure by converting between moles and volume using `1 mol = 24 dm³`, then applying the mole ratio from the balanced equation.