Experimental errorsExperimental errors

Key terms-Key terms-

PrecisionPrecision – If you work carefully and use – If you work carefully and use apparatus correctly your measurements should apparatus correctly your measurements should be precise i.e. repeated measurements should be precise i.e. repeated measurements should be close to together. The degree of precision is be close to together. The degree of precision is limited by the apparatus you uselimited by the apparatus you useAccuracyAccuracy – the accuracy of your measurement is – the accuracy of your measurement is how close it is to the ‘true’ valuehow close it is to the ‘true’ valueAnomalous readingsAnomalous readings – one that does not fit the – one that does not fit the pattern of other repeated measurements. They pattern of other repeated measurements. They should be ignored. Explain clearly why you should be ignored. Explain clearly why you consider it anomalous and give reasons for the consider it anomalous and give reasons for the reading if possiblereading if possible

ErrorsErrors

Can affect both the reliability and accuracy of Can affect both the reliability and accuracy of resultsresults

Various types-Various types-– Procedural errorsProcedural errors – e.g. adding to much acid near the – e.g. adding to much acid near the

end point of a titration. These are errors but can be end point of a titration. These are errors but can be minimised with good techniqueminimised with good technique

– Apparatus errorApparatus error – this is the source of error – this is the source of error associated with each piece of apparatus used. These associated with each piece of apparatus used. These kind of errors are unavoidable but can be minimised kind of errors are unavoidable but can be minimised by use of appropriate equipment. They can be by use of appropriate equipment. They can be quantified.quantified.

Calculating errorsCalculating errors

For example – use of a 25cmFor example – use of a 25cm33 pipette, the pipette, the error is 0.05 cmerror is 0.05 cm33..

So the volume could be as large as So the volume could be as large as 25.05cm25.05cm33 or as small as 24.95cm or as small as 24.95cm33

Can be written as 25 0.05cm3Can be written as 25 0.05cm3

Percentage error is often more useful-Percentage error is often more useful-

Percentage error= (error x 100)/readingPercentage error= (error x 100)/reading

+-

Example-Example-

Calculate the percentage error when 4cmCalculate the percentage error when 4cm33 of water is measured out using-of water is measured out using-

A 50cm3 measuring cylinder (error +-1cm3)A 50cm3 measuring cylinder (error +-1cm3)A 10cm3 measuring cylinder (error +-0.2cm3)A 10cm3 measuring cylinder (error +-0.2cm3)A graduated pipette (error +- 0.1cm3)A graduated pipette (error +- 0.1cm3)

Calculate the error when weighing out Calculate the error when weighing out 0.4g of a solid using-0.4g of a solid using-

A balance accurate to 1 decimal place (error+-A balance accurate to 1 decimal place (error+-0.05g)0.05g)A balance accurate to 2 decimal placesA balance accurate to 2 decimal places

Errors in differences-Errors in differences-

Example – using a burette or thermometerExample – using a burette or thermometerThe error in each reading on a burette is The error in each reading on a burette is

+-0.05cm+-0.05cm33

Therefore the error in the difference is +-0.10cmTherefore the error in the difference is +-0.10cm33

Similarly the error in measuring an Similarly the error in measuring an increase or decrease in temperature is increase or decrease in temperature is twice the error of each individual readingtwice the error of each individual reading

The final apparatus errorThe final apparatus error

To calculate the final %error in a reading add To calculate the final %error in a reading add together the %errors for each of the individual together the %errors for each of the individual pieces of equipment usedpieces of equipment usedExample-Example-

What is the %error in the concentration of a sodium What is the %error in the concentration of a sodium hydroxide solution made up by-hydroxide solution made up by-

1.1. Weighing out 10g of NaOH pellets on a 1 decimal place Weighing out 10g of NaOH pellets on a 1 decimal place balancebalance

2.2. Making the solution up in a 250cmMaking the solution up in a 250cm33 volumetric flask (error volumetric flask (error +-0.15cm+-0.15cm33))How would the % error change if a 3d.p balance was used How would the % error change if a 3d.p balance was used instead?instead?

And finally……And finally……

It is often useful to compare the It is often useful to compare the experimentally determined value with an experimentally determined value with an actual (or target value)actual (or target value)% difference = (actual-experimental)/actual X 100% difference = (actual-experimental)/actual X 100

This should be compared to the apparatus This should be compared to the apparatus error and the differences explained!error and the differences explained!