AQA A-Level Chemistry Amount of Substance Questions and Answers

AQA A-Level Chemistry Amount of Substance Questions and Answers What is the relative atomic mass? the ratio of the average mass of one atom of an element to one twelfth of the mass of an atom of carbon-12 (Oxford Languages). How do you work out the relative atomic mass? Why is carbon-12 so significant? Carbon-12's atomic mass could be measured particularly accurately compared to the other elements on the periodic table. Historically, chemists and physicists used oxygen to describe RAM and RFM but they used different isotopes so they compromised on using Carbon-12 as it was in between (Naomiisbell Chemistry Blog) What is relative molecular mass? the ratio of the average mass of one molecule of an element or compound to one twelfth of the mass of an atom of carbon-12. How do you work out relative molecular mass? Add up the relative atomic masses? What is the relative formula mass? Relative molecular mass but for ionic compounds as they aren't really molecules. What is an entity? A molecular entity, or chemical entity, is "any constitutionally or isotopically distinct atom, molecule, ion, ion pair, radical, radical ion, complex, conformer, etc., identifiable as a separately distinguishable entity What is the avogrado's constant? The number of atoms or molecules in one mole of a substance, equal to 6.023 × 10^23. What is the mole? A mole of a substance or a mole of particles is defined as exactly 6.×10²³ particles, which may be atoms, molecules, ions, or electrons. How do you calculate moles? What is a solution? A homogeneous mixture of two or more substances What are the units of concentration? 1 mol dm^-3 How do you get 1mol dm^-3 of solution? Add the 1dm^3 solvent to 1mol of solute, NOT THE OTHER WAY ROUND. How do you work out the number of moles in a given volume of solution? Moles = concentration (mol dm^-3) x volume (dm^3) (a dm is 1000cm^3) What is Boyle's Law pressure x volume = constant What is Charles' Law? volume is directly proportional to temperature volume / temperature = constant What is Gay-Lussac's Law? pressure is directly proportional to temperature pressure / temperature = constant What is the ideal gas equation? Pressure (in pascals) x volume (m^3) = number of moles x gas constant (J K^-1 mol^-1) x Temperature (K) Acts as if the gas is perfect but not all gasses are ideal and work like this (only works in room temperature). How do you work out the relative molecular mass of a gas? Rearrange to find the moles of the gas and then find the mass of one mole. What is the empirical formula? An empirical formula is the simplest ratio of whole number atoms of each element in a compound. How do you work out the empirical formula? 1) List the elements in the compound 2) Write underneath their experimental masses 3) Divide the mass of each element by its Ar 4) Put the numbers into a ratio 5) Simplify the ratio What is the molecular formula? A molecular formula is the actual number of atoms of each element in a molecule How do you work out the molecular formula? Find the empirical formula out. Then, divide the Mr needed by the empirical Mr and multiply the ratio by that number. What is on the left-hand side of an equation? Reactants What is on the right-hand side of an equation Products What is stoichiometry? The relationship between the relative quantities of substances taking part in a reaction or forming a compound, typically a ratio of whole integers. What is the stoichometric relationship? Terms like 'mole ratio'. How do you balance equations? By adding a coefficient number in front of a substance What is (s) Solid What is (g) Gas What is (l) Liquid What is (aq) Aqueous (dissolved in water) How do you write ionic equations? 1. Write out all ions 2. Cross out spectator ions e.g. BaCl2 + Na2SO4 - 2NaCl + BaSO4 Ba (2+) + 2Cl (-) + 2Na (+) + SO4 (2-) - 2Na (+) + 2Cl (-) + BaSO4 = Ba (2+) + SO4 (2-) - BaSO4 What is a spectator ion? An ion that appears on both sides of an equation and is not directly involved in the reaction How do you do a titration? -Measure a set volume of alkali into a flask -Line the burette with an acid of known concentration, then fill the burette with the same acid -Slowly add the acid to the flask, making sure to swirl it at regular intervals -When the solution changes colour, measure the volume of acid used -To get a pure solution, repeat this process with no indicator What is the atom economy? Atom economy is a measure of the proportion of reactant atoms that become part of the desired product (rather than by-products) in the balanced chemical equation. How do you calculate percentage atom economy? What is the percentage yield? The amount of product you get Why is a higher atom economy better? Efficient processes have high atom economies, and are important for sustainable development, as they use fewer natural resources and create less waste How do you work out percentage yield? actual yield/theoretical yield x 100 How do you calculate the maximum theoretical yield? The RFM of the products in the equation.