Moles

• Chemical formulae, equations and calculations are a part of Principles of chemistry.
• Word equations and balanced chemical equations can be written for reactions studied in this specification, and for unfamiliar reactions where suitable information is provided.
• In a chemical equation, 'g' means gas, 's' means solid, 'l' means liquid, 'aq' means aqueous.
• An example of a word equation is hydrochloric acid + sodium hydroxide -> sodium chloride + water.
• An example of a balanced chemical equation is HCl + NaOH -> NaCl + H 2 O.
• To balance an equation, you need to make sure there are the same number of each element on each side of the equation and if there isn’t use big numbers at the front of a compound to balance it.
• Relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.
• In a balanced chemical equation, the sum of Mr of reactants in quantities shown equals the sum of Mr of products in quantities shown.
• The mole (mol) is the unit for the amount of a substance.
• Chemical amounts are measured in moles, therefore it is the amount of substance.
• Moles can be converted to grams by using the equation: moles = mass ÷ relative atomic mass or mass = moles x relative atomic mass.
• If given the reacting mass of one reactant and asked to find the mass of one product formed, find moles of that one substance: moles = mass /molar mass.
• Use balancing numbers to find the moles of desired reactant or product.
• Masses of reactants & products can be calculated from balanced symbol equations.
• The symbol for the unit mole is mol.
• One mole of a substance contains the same number of the stated particles, atoms, molecules or ions as one mole of any other substance.
• Calculations involving amount of substance, relative atomic mass (Ar) and relative formula mass (Mr) can be carried out.
• Chemical equations can be interpreted in terms of moles.
• For example, Mg + 2HCl → MgCl2 + H2 shows that 1 mol Mg reacts with 2 mol HCl to produce 1 mol MgCl2 and 1 mol H2.
• The number of moles in 42g of carbon is calculated as: Moles = Mass / Mr = 42/12 = 3.5 moles.
• The mass of one mole of a substance in grams is numerically equal to its relative formula mass.
• For example, the Ar of Iron is 56, so one mole of iron weighs 56g.
• The Mr of nitrogen gas (N 2 ) is 28 (2x14), so one mole is 28g.
• 2NaOH + Mg → Mg(OH)2 + 2Na, if you had 2 moles of Mg, you would form 2x2=4 moles of Na.
• Understand how the formulae of simple compounds can be obtained experimentally, including metal oxides, water and salts containing water of crystallisation.
• Maximum amount of product possible is not always possible to obtain the calculated amount of a product for 3 reasons: reaction may not go to completion because it is reversible, some of the product may be lost when it is separated from the reaction mixture, and some of the reactants may react in ways different to the expected reaction.
• Experiment to find formula of magnesium oxide: weigh some pure magnesium, heat magnesium to burning in a crucible to form magnesium oxide, as the magnesium will react with the oxygen in the air, weigh the mass of the magnesium oxide, known quantities: mass of magnesium used & mass of magnesium oxide produced, required calculations: mass oxygen = mass magnesium oxide - mass magnesium, moles magnesium = mass magnesium ÷ molar mass magnesium, moles oxygen = mass oxygen ÷ molar mass oxygen, calculate ratio of moles of magnesium to moles of oxygen, use ratio to form empirical formula.
• Molecular formula is the number of atoms of each element in a compound.
• Percentage yield = Amount of product produced x 100.
• Yield is the amount of product obtained is known as yield.
• Empirical formula is the simplest whole number ratio of atoms of each element in a compound.
• Mass = moles x molar mass(of the product) to find mass.
• Volume of 1 mol of any gas at RTP (room temperature and pressure: 20 degrees C and 1 atmosphere pressure) is 24 dm 3.
• This sets up the equation: Volume (dm 3 ) of gas at RTP = Mol x 24.
• Use this equation to calculate the volumes of gaseous reactants and products at RTP.
• Equal amounts in mol of gases occupy the same volume under the same conditions of temperature and pressure (e.g RTP).
• Molecular formula from empirical formula and relative molecular mass.
• If answer was 2 and the empirical formula was Fe 2 O 3 then the molecular formula would be empirical formula x 2 = Fe 4 O 6.
• Fe 2 O 4 , the empirical formula is the simplest whole number ratio, which would be FeO 2 ● if there is no common multiple, you already have the empirical formula.
• To calculate concentration of a solution use the equation concentration (g dm -3 ) = mass of solute (g) ¨ volume (dm 3 ).