titrations and volumetric calculations

what are volumetric calculations?

calculations about reactions involving solutions

units of concentration

mol/dm³

formula for concentration

concentration = moles / volume (dm³)

how do you make up a standard solution?

accurately weigh out … g of substance (work out mass you need for desired conc using formula)

dissolve solid in about 50cm³ of deionised water in a beaker

transfer solution to a 250cm³ volumetric flask

rinse beaker with deionised water and add washings to flask

stopper flask and invert it 15 times to ensure mixing

add more deionised water to make volume up to 250cm³ (ensure bottom of meniscus is on the measuring line)

stopper and invert a few more times

what do we use titrations for?

working out concentration of an ‘unknown’ solution by comparing it with a solution whose concentration is known

how do you carry out a titration?

use a pipette to transfer 25cm³ of acid/alkali into a conical flask

fill a burette with 50cm³ of alkali/acid (whichever you didn’t use in conical flask), ensuring bottom of meniscus is on measuring line

add a few drops of phenophthalein indicator to conical flask (colourless in acid, magenta in alkali)

add acid/alkali from burette to conical flask until colour changes (be specific about what the colour change is)

add dropwise towards the end

swirl flask whilst adding it

record titre volume

repeat until you get concordant results