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neurologyYou need to know

• The law of conservation of mass states that no atoms are lost or made during 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 atoms of each element on both sides of the equation.
• The large numbers in front of formulae in a balanced equation are coefficients, and they show the ratio of particles or moles reacting and being formed.
• Chemical formulae and element symbols must not be changed when balancing an equation; only coefficients can be added or changed.
• Some reactions seem to change mass in an open container because a gas may escape from the container or a gas from the air may react and be gained.
• A mole is an amount of substance containing `6.02 \times 10^{23}` particles, which is the Avogadro constant.
• The particles counted by a mole can be atoms, molecules, ions or formula units, depending on the substance.
• The relative atomic mass, `A_r`, is the mean mass of an atom of an element compared with one twelfth of the mass of an atom of carbon-12.
• The relative formula mass, `M_r`, is the sum of the relative atomic masses of all the atoms shown in a chemical formula.
• The mass of one mole of atoms is the relative atomic mass in grams, and the mass of one mole of a compound is the relative formula mass in grams.
• The amount in moles is calculated using `n = m \div M_r`, where `n` is moles, `m` is mass in grams, and `M_r` is relative formula mass.
• The mass of a substance is calculated using `m = n \times M_r`, where `m` is mass in grams, `n` is moles, and `M_r` is relative formula mass.
• Relative atomic mass can be calculated from isotope data using the weighted mean: sum of `isotopic mass \times percentage abundance` divided by 100.
• A balanced equation can be interpreted in moles; for example, `Mg + 2HCl \rightarrow MgCl_2 + H_2` means 1 mole of magnesium reacts with 2 moles of hydrochloric acid.
• In a reaction, the limiting reactant is the reactant that is used up first and controls the maximum amount of product that can form.
• An excess reactant is present in a greater amount than needed to react completely with the limiting reactant, so some of it remains after the reaction.
• The coefficients in a balanced symbol equation can be found from reacting masses by converting grams to moles and then simplifying the mole ratio to whole numbers.
• The concentration of a solution in grams per cubic decimetre is the mass of solute dissolved in each cubic decimetre of solution.
• Mass concentration is calculated using `concentration = mass \div volume`, with mass in grams, volume in cubic decimetres, and concentration in grams per cubic decimetre.
• One cubic decimetre is equal to 1000 cubic centimetres, so a volume in cubic centimetres must be divided by 1000 to convert it to cubic decimetres.

• Balance a chemical equation by counting atoms of each element and adding coefficients until both sides contain equal numbers of each atom.
• Calculate an unknown mass in a reaction using conservation of mass by subtracting the known masses from the total mass on the other side of the equation.
• Explain an apparent change in mass by identifying whether a gaseous reactant or product has entered or left an open reaction container.
• Calculate relative formula mass by adding the relative atomic masses for every atom in the formula, including atoms inside brackets multiplied by the outside number.
• Calculate the number of moles or the mass of a substance using `n = m \div M_r` or `m = n \times M_r` and correct units.
• Calculate relative atomic mass from isotope abundances by multiplying each isotope mass by its percentage abundance, adding the results, and dividing by 100.
• Use a balanced equation to calculate the mass of an unknown reactant or product by converting known mass to moles, applying the mole ratio, and converting back to mass.
• Identify the limiting reactant by comparing the available moles of each reactant with the mole ratio required by the balanced equation.
• Deduce the coefficients in a balanced symbol equation from reacting masses by converting each mass to moles and simplifying to the smallest whole-number ratio.
• Calculate the mass of solute, volume of solution or concentration using `concentration = mass \div volume`, converting between cubic centimetres and cubic decimetres when needed.