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neurologyYou need to know

• The law of conservation of mass states that mass is neither created nor destroyed during a chemical reaction because atoms are rearranged, not created or destroyed.
• In a closed system, no substances enter or leave, so the total mass of the reactants equals the total mass of the products.
• A reaction can appear to lose mass if a gaseous product escapes, such as carbon dioxide released when an acid reacts with a carbonate.
• A reaction can appear to gain mass if a gas from the surroundings reacts with the substances being weighed, such as oxygen combining with magnesium during burning.
• A symbol equation shows reactants on the left and products on the right, with chemical formulae identifying the elements and the numbers or ratios of atoms in each substance.
• A balanced equation has equal numbers of atoms of each element on both sides, achieved by changing the coefficients before formulae rather than the subscripts within them.
• Relative atomic mass is the weighted mean mass of an atom of an element compared with one-twelfth of the mass of a carbon-12 atom.
• Relative atomic mass is calculated from percentage isotope abundances using `A_r = \frac{\sum(\text{isotope mass} \times \text{percentage abundance})}{100}`.
• Relative formula mass is the sum of the relative atomic masses of all the atoms shown in a chemical formula, and both relative atomic mass and relative formula mass have no units.
• A mole is an amount of substance containing approximately `6.02 \times 10^{23}` specified particles, which may be atoms, molecules, ions, or formula units.
• The Avogadro constant is the number of particles per mole and has an approximate value of `6.02 \times 10^{23}\ \text{mol}^{-1}`.
• The mass of one mole of a substance in grams is numerically equal to its relative atomic mass or relative formula mass, and its molar mass is expressed in grams per mole.
• Amount in moles, mass, and molar mass are related by `n = \frac{m}{M}`, where mass is measured in grams and molar mass in grams per mole.
• The coefficients in a balanced chemical equation give the mole ratios in which reactants react and products form, not their mass ratios.
• The equation `\mathrm{Mg + 2HCl \rightarrow MgCl_2 + H_2}` shows that one mole of magnesium reacts with two moles of hydrochloric acid to produce one mole of magnesium chloride and one mole of hydrogen.
• The limiting reactant is used up first and determines the maximum amount of product that can form.
• An excess reactant remains after the limiting reactant is used up, so adding more of the excess reactant alone does not increase the amount of product.
• Equation coefficients can be found from reacting masses by converting each mass to moles and expressing the mole amounts as the simplest whole-number ratio.
• Mass concentration is the mass of dissolved solute per unit volume of solution, given by `c = \frac{m}{V}`, so increasing solute mass at fixed volume increases concentration while increasing volume at fixed solute mass decreases it.
• A concentration in grams per cubic decimetre requires solution volume in cubic decimetres, with `1\ \text{dm}^{3} = 1000\ \text{cm}^{3}`.

• Calculate an unknown reactant or product mass using conservation of mass, including the masses of any gases that enter or leave the measured system.
• Balance symbol equations using the smallest whole-number coefficients, checking the atom count for every element without changing chemical formulae.
• Calculate relative atomic mass from isotope masses and percentage abundances by finding the weighted mean.
• Calculate relative formula mass using periodic table values, accounting correctly for subscripts and brackets in chemical formulae.
• Calculate amount in moles from mass and molar mass, or mass from amount in moles and molar mass, giving the appropriate units.
• Calculate an unknown reacting mass or product mass by converting the known mass to moles, applying the balanced equation's mole ratio, and converting back to grams.
• Identify the limiting and excess reactants by comparing each available mole amount with its equation coefficient, then calculate the maximum product amount from the limiting reactant.
• Deduce equation coefficients from masses of reactants and products by converting masses to moles and reducing the amounts to a whole-number ratio that balances the equation.
• Calculate concentration, solute mass, or solution volume using mass concentration, converting between cubic centimetres and cubic decimetres where necessary.