Paper 1B stuff

crystallisation - less mass lost than expected

• reason: not heating long enough

• solution: heat to constant mass

crystallisation - more mass lost than expected

• reasons:

  • spitting of solid out of crucible during heating

  • impurities in crucible burnt away during heating

  • stirring of solid during heating (if given in method), solid left on spatula/stirring rod

• respective solutions:

  • use a larger crucible/lower mass of solid heated

  • check/clean crucible beforehand

  • don’t stir it

how to make a standard solution

• weigh out (certain mass) of named solid to 2 decimal places

• Fully dissolve in a beaker using distilled water

• Transfer into a 250cm³ volumetric flask (or other size if given)

• Transfer all rinsings

• Add distilled water up to the meniscus

• Mix and invert solution 20 times to get a homogenous solution

justification - making a standard solution

• using distilled water - it does not contain dissolved ions

• transfer rinsings - so all ions are transferred and not left behind

• volumetric flask - individually calibrated and accurate containers of volume/solution

• inverting 20 times - to produce homogenous solution so concentration is the same throughout

• adding distilled water below meniscus - to account for any volume changes caused by dissolved solids

errors

• systematic - causes inaccurate results in the same direction

• random error - causes inaccurate results in both directions