Topic 5 Formulae, Equations and amounts of a substance

It is a way of measuring the amount of a substance in chemistry contains 6.02 x 10^23 atoms/molecules

Number of particles = Avogadro’s number x number of moles

Moles = mass/Mr

Moles = Concentration x volume. Make sure volume is in dm3 cm3 to dm3 is /1000

Conc = Mass / Volume

It is the simplest whole number ratio of elements in a compound • Write out the elements involved • Write the % as masses • Divide these by Mr to get the number of moles • Divide all the numbers by the smallest number of moles

It shows the ions that are formed in solution and shows which particles are reacting. Only take the ions of the aqueous substances H2SO4 + 2KOH —-> K2SO4 + 2H2O 2H+ + SO42- + 2K+ + 2OH- —> 2K+ + SO42- +2H2O Cancel any ions that appear on both sides of the equation 2H+ + 2OH- —-> 2H2O

Balanced equations can be used to calculate theoretical masses How much CaO can be made when 34g of Ca is burnt completely in oxygen? 1. Write out the equation and balance it 2. Work out Mr of species involved. Write these as mass in grams 3. Divide the Ca side by 80 to find 1g then multiply by 34 to get 34g. Do the same for the CaO side

These are reactions where a more reactive element takes the place of a less reactive element in a compound F2(aq) + 2NaBr(aq) —-> 2NaF(aq) + Br2(aq) F2(aq) + 2Br-9aq) —-2F-(aq) + Br2(aq)

An acid and a base will form a salt and water. Carbonates will produce CO2 too. Potassium nitrate HNO3(aq) + KOH(aq) —-> KNO3(aq) + H2O(l) H+(aq) + OH-(aq) —–> H2O(l)

When a solid is produced after 2 aqueous reactants react is called a precipitation reaction BaCl2(aq) + K2SO4(aq) ——> 2KCl(aq) + BaSO4(s) Ba2+(aq) + SO42-(aq) —–> BaSO4(s)

Gases occupy the same volume of space under the same conditions Number of moles = Volume(dm3) / Molar gas volume. Molar Gas volume at standard room temp and pressure is 24dm3mol-1 Molar gas volume at standard temp and pressure is 22.4dm3mol-1

pV=nRT p = pressire in pascals V = volume in m3 n = moles in mol R = Gas constant (8.31) K = Temperature in Kelvins

M to dm is x10 dm to cm is x 10 m2 to dm2 is x 100 dm2 to cm2 is x 100 m3 to dm3 is x 1000 dm3 to cm3 is x1000

Hazards are things that could cause harm to you or others around you

Risks is the likelihood of the hazard causing harm to you or something else Risks in experiments can range from: Scalding from hot liquids Burns from hot equipment Burns from chemicals being used Cuts from glass or metals Harm or poisoning from gases and chemicals Risk assessments should be carried out prior to any experiment being done. It needs to cover - Identification of all hazards The risk associated with each hazard The likelihood of something going wrong The seriousness of the risk How to reduce the risk

They are made from solids with a known mass dissolved in water to a fixed volume in a volumetric flask. We can work out the concentration f this too

• Weigh out the amount of solid precisely using a balance and plastic/glass weighing boat • Transfer the solid from the weighing boat to a beaker. Wash any solid left behind into the beaker using deionised water. • Dissolve your solid fully using deionised water. Stir to ensure the solid is fully dissolved. • Transfer the solution to a volumetric flask. Use the funnel to avoid spillage and rinse the beaker and glass ro d into the flask to make sure most of the solution is transferred • Use more deionised water to fill of the graduation line. Be careful not to go above this line! Use a pipette to fill to the line when you get near. • Invert the flask a few times. Make sure you put a lid on it first! This ensures your solution is thoroughly mixed and ready to use

• Work out the number of moles of the solution you want to make • Use the number of moles from the previous step to calculate the mass

• Have acid or alkali in burette with a known concentration • Have an acid or alkali with an unknown concentration but a known volume in the conical flask. Add a few drops of indicator too • Add the chemical in the burette to the conical flask until the indicator changes colour. This is known as the endpoint. Add drop by drop near endpoint • Read how much chemical was added from the burette to neutralise the chemical in the conical flask. Read the bottom of the meniscus. Always read at eye level • Record your results to 2 d.p and repeat until you get 2 results that are concordant (within 0.10cm3 of eachother)

Phenolphthalein in acid = colourless in alkali = pink Methyl orange In acid = yellow In alkali = red

• Write out the equation and balance it • Calc the number of moles of HCl • Use the equation ot find out the molar ratio in order to work out the moles of KOH • Now calc concentration by using conc = moles / volume

Uncertainty is the degree of error your measurements have with a piece of equipment The uncertainty is half the smallest increment the equipment can measure In a burette there are marks every 0.1 cm3. You can read this burette to the nearest 0.05cm3 as any reading you take wouldn’t be more than 0.05cm3 out. We can say the uncertainty of the reading will have a maximum error of +- 0.05cm3 If you are using more than one measurement with the same units you need to add the uncertainties

% error = Uncertainty x by the number of measurements / Reading x 100 You can reduce % error by using more precise equipment. Also by using larger volumes of liquid