quantitative chemistry

• the law of conservation of mass: total mass of reactants = total mass of products
• to calculate the relative formula mass of a compound, we need to know the relative atomic masses of the elements that make the compound, then we multiply the Ar values by the number of atoms of that element in the compound, then we add these numbers together
• in chemical reactions, substances may seem to lose of gain mass. normally one of the reactants is a gas, and gas can escape into the surrounding environment
• when wood is burnt, it releases carbon dioxide and water, which can escape into the atmosphere but can't be weighed easily at the end of a reaction
• in corrosion of metals, the mass of the oxide produced will be larger that the mass of the starting metals because oxygen atoms from the atmosphere have ben added. the change in mass will equal the mass of the oxygen atoms which have been added
• during the reaction of zinc +hydrochloric acid, you would expect the mass of the flask and reactants to decrease because the hydrogen gas produced will escape from the flask
• every chemical measurement involves some uncertainty
• the larger the uncertainty, the larger the range of results you should expect. this is the reason why it is important to repeat the measurements from experiments several times, and calculate an average from them
• chemical equations must be balanced to make sure that there are the same number of atoms of each element before and after a reaction
• the molecular formula shows the actual amount of atoms which make up a molecule, whereas the empirical formula shows the simplest ratio of atoms which make up a molecule
• moles are a measurement of chemical amounts
• because atoms and molecules are so small, it is inconvenient to talk about individual atoms, instead chemical amounts can be measured in moles
• 1 mole of a substance is 6.02 x 10^23 atoms of that substance, known as the avagadro constant
• the relative formula mass of a substance is equal to the mass of 1 mole of that substance - for an element, this is the same as the relative atomic mass
• moles = mass / relative formula mass
• in many chemical reactions, it is important to use up a reactant completely
• the reactant that is completely used up is called the limiting reactant because the reaction stops when it is used up
• it's important that no acid remains after the reaction, which is achieve by using an excess amount of the other reactant
• the percentage yield for any reaction is always less than 100%
• an incomplete reaction can happen because the reaction is very slow and hasn't been given enough time to happen, or the reaction is reversible and some of the products have been converted back into reactants
• some of the chemicals will remain stuck to the apparatus and will be wasted
• it might be difficult to fully separate the product from the reaction mixture
• unexpected reactions (side reactions) can happen, potentially meaning the intended product is not created
• the percentage yield of a reaction is calculated as the real yield divided by the theoretical yield multiplied by 100
• the real yield is the mass if a desired product obtained from a reaction
• the theoretical yield is the maximum mass of a product that could possibly be created from a reaction, which can be calculated from a balanced equation if we know the mass of the reactants
• reaction pathways with high atom economies are good because using the smallest possible amount of reactants saves money
• reaction pathways with high atom economies are good because it means less waste is produced
• when selecting a reaction pathway, we don't just consider atom economy, we also look at how much the reactants cost, what the percentage yield will be, how quickly the reaction will happen, how useful the by-products of the reaction are, and the position of equilibrium