OCR A level Chemistry definitions | Questions and Answers with complete solution

Relative isotopic mass - The mass of an isotope relative to 1/12 of the mass of a carbon-12 atom. Relative atomic mass Ar - The weighted mean mass of an atom of an element relative to 1/12 of the mass of a carbon-12 atom. Weighted: The percentage abundance of the isotopes of that element Mean: The relative isotopic mass of each isotope What is a mole? - The amount of substance containing as many particles as there are atoms in 12g of the carbon-12 isotope. This is 6.02 x 10^23 particles. Molar Mass - The mass of one mole of a substance in grams. Note: Yes, this IS the same number as the relative atomic mass, but the definitions are very different. Empirical formula How is this worked out? - The simplest whole number ratio of atoms of each element in a compound. To work out: Find how many moles are in the masses of each element (these are provided in q), then do some ratio fiddling. Molecular formula - The actual number of atoms of each element in a molecule. To work out: divide the Mr of the empirical formula by the actual Mr to find out how many formula units of the empirical formula there are in the compound. Then multiply the empirical formula by this number.e.g. if e.f. is CH20 and the Mr is 180 Mr CH20 = 30. --- 180/30 = 6 --- 6(CH2O) = C6H12O6 Percentage yield - The actual yield divided by the theoretical yield x100% What factors can lower the percentage yield? - -Reaction is reversible and therefore at equilibrium, the yield of products is not 100% - Reaction may not have compltely occurred - Reactants may be impure - Purification of the product could result in loss of some mass (e.g. in filtering) Atom economy - Measure of how well atoms have been utilised. Sum of molar masses of desired products/ Sum of molar masses of all products x100% High atom economy = high proportion of reactant atoms are in a useful product. Produce small proportion of waste products. Addition reactions/ reactions where all the products are useful- atom economy = 100% Ionic bond - Strong electrostatic force of attraction between oppositely charged ions Covalent bond - The strong electrostatic force of attraction between a shared pair of electrons and the nuclei of the bonded atoms Polar covalent bond - One where the bonded electron pair are not shared equally between bonded atoms. Due to differences in electronegativity between bonded atoms.Hydrogen bond - the intermolecular force occurring when a hydrogen atom that is bonded to a N, O or F atom of one molecule is attracted to a lone pair of electrons on the N, O or F atom of another molecule.