Chemistry Calculations All the chemistry calculations for AQA Additional Chemistry 2.5 / 5 based on 3 ratings ? ChemistryCalculations, moles and yieldGCSEAQA Created by: Rachel GregoryCreated on: 06-06-08 10:29 Mass number, atomic number, atomic mass, formula m Mass number = total number of protons + neutrons Atomic number = number of protons Isotopes contain the same number of protons and electrons as the atom, but different numbers of neutrons Relative atomic mass = mass number Relative formula mass = all mass numbers added together in formula Relative formula mass = one mole of that substance 1 of 8 Percentage composition Percentage composition = how much of an element there is in the compound 1. Work out relative formula mass 2. Divide relative atomic mass of element by formula mass of compound 3. Turn into a percentage (x100) 4. If there are more than one of the atoms in the compound multiply answer by the number of atoms 2 of 8 Conservation of mass Mass is never lost or gained in reactions – conservation of mass This means you can work out the mass of one substance if the others are known BUT Reversible reactions may not go to completion Some product may be lost Some reactants may react in an unexpected way 3 of 8 Atom economy Atom economy is the percentage of products that are useful, worked out like percentage composition High atom economy = good for sustainable development Mass of desired products ÷ mass of reactants x 100 4 of 8 Concentration of a solution Measured in moles/cubic decimetre The greater the concentration the more dissolved particles there are in the solution 5 of 8 Volumes of gases A larger volume contains a larger (proportionally) amount of atoms One mole of any gas at room temperature occupies approx 24 cubic decimetres 6 of 8 The Empirical Formula Used to find formulae of compounds from weights of reactants 1. Find the mass of each element 2. Find the relative atomic mass of each element 3. Divide mass of each element by relative atomic mass 4. Divide results by smallest number to give number of each elements in compound 5. Number of atoms tells you formula 7 of 8 Percentage yield In some reactions not all of the products are made The yield is the actual amount obtained Percentage yield is actual mass obtained ÷ maximum theoretical mass x 100 8 of 8
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