2 Key terms-Precision – If you work carefully and use apparatus correctly your measurements should be precise i.e. repeated measurements should be close to together. The degree of precision is limited by the apparatus you useAccuracy – the accuracy of your measurement is how close it is to the ‘true’ valueAnomalous readings – one that does not fit the pattern of other repeated measurements. They should be ignored. Explain clearly why you consider it anomalous and give reasons for the reading if possible
3 Errors Can affect both the reliability and accuracy of results Various types-Procedural errors – e.g. adding to much acid near the end point of a titration. These are errors but can be minimised with good techniqueApparatus error – this is the source of error associated with each piece of apparatus used. These kind of errors are unavoidable but can be minimised by use of appropriate equipment. They can be quantified.
4 Calculating errorsFor example – use of a 25cm3 pipette, the error is 0.05 cm3.So the volume could be as large as 25.05cm3 or as small as 24.95cm3Can be written as cm3Percentage error is often more useful-Percentage error= (error x 100)/reading+-
5 Example-Calculate the percentage error when 4cm3 of water is measured out using-A 50cm3 measuring cylinder (error +-1cm3)A 10cm3 measuring cylinder (error +-0.2cm3)A graduated pipette (error cm3)Calculate the error when weighing out 0.4g of a solid using-A balance accurate to 1 decimal place (error+-0.05g)A balance accurate to 2 decimal places
6 Errors in differences- Example – using a burette or thermometerThe error in each reading on a burette is+-0.05cm3Therefore the error in the difference is cm3Similarly the error in measuring an increase or decrease in temperature is twice the error of each individual reading
7 The final apparatus error To calculate the final %error in a reading add together the %errors for each of the individual pieces of equipment usedExample-What is the %error in the concentration of a sodium hydroxide solution made up by-Weighing out 10g of NaOH pellets on a 1 decimal place balanceMaking the solution up in a 250cm3 volumetric flask (error cm3)How would the % error change if a 3d.p balance was used instead?
8 And finally……It is often useful to compare the experimentally determined value with an actual (or target value)% difference = (actual-experimental)/actual X 100This should be compared to the apparatus error and the differences explained!