C4 Chemical calculations

• The relative atomic mass is the average mass of its atoms compared to its isotope
• Relative formula mass is the sum of atomic masses in a compound
• We use relative masses because the actual masses are so small it would be difficult to use these values
• Moles is the unit of measurement that tells us how many particles are in a substance
• 1 mole contains 6x10^23 particles
• Moles= mass(g)/relative mass
• in a reaction the total mass of the products is equal to the total mass of reactants
• One of the reactants will be in excess so there is more of it than is needed so it isn’t a limiting reactant
• The empirical formula tells you the number of each atom in a compound
• The empirical formula is the simplest ration of elements in a compound
• Atom economy is a measure of the amount of starting materials that become useful products
• Atom economy= mr of useful product/ total mr of products
• High atom economy has little waste and low costs
• low atom economy has more waste so a higher cost
• 1dm3 = 1000cm3
• Concentration of a solution can be increased by increasing the mass of solute in the same volume or by evaporating some of the solvent
• most reactions don't give 100% yield because unexpected products can be made, reaction may not be complete, the reaction may be reversible or there may be product left on the apparatus
• 1 mole of any gas at room temperature and pressure has a volume of 24dm3
• 24dm3 is known as the molar gas volume
• when a substance is dissolved in water an aqueus solution is formed
• acids contain hydrogen ions (h+)
• alkalis are soluble bases that contain hydroxide ions (OH-)
• bases neutralise acids and they include metal oxides and metal hydroxides
• a pH <7 is an acidic solution
• a pH >7 is an alkali solution
• indicator can be used to measure the pH scale (universal indicator or phenolphthalein)
• a more accurate way to measure pH is using a pH probe
• titrations are used to calculate the concentration of solutions
• titrations are very precise and accurate so special equipment is used
• titrations use volumetric pipets to measure and transfer specific volumes and burettes are used to measure the addition of one solution to another
• 1. use a volumetric pipet to transfer 25cm2 of sodium hydroxide solution into the conical flask and stand the flask on a white tile 2. fill the burette with dilute sulphuric acid to the 0cm3 line 3. put 3-6 drops of methyl orange indicator into the the conical flask 4. titrate the sulphuric acid into the flask (constantly swirling the flask whilst adding) 5. close the tap when the colour change becomes apparent 6. record the volume of acid you added and repeat until you have concordant results and calculate the mean titre value
• a pH curve or titration curve shows how the pH changes during neutralisation
• the equivalence point is the volume of acid or alkali needed to neutralise
• titre value = final titre-initial titre
• in titrations all data must be recorded to 2dp
• c = n x 1000 / v