Calculating and Interpreting Error in AS Chemistry

Click on the speaker icon on the right to hear the sound file

You will need to be able to here the sound files to make any sensible use of this resource.

Print a copy of the worksheet which is available here: worksheet

Sources of Error 1

Reading the scale:

Usually a scale can be read to 0.5 division

Sources of Error 2

Digital instruments have error in reading the scale:-

5 in the next decimal place

The reading is 2.37 seconds with an uncertainty of 0.005 seconds

Sources of Error 3

Manufacturing / Calibration error

Has the scale been printed in the right place?

Does the stopclock run at the correct speed?

In Chemistry it is normal to find the error written on the measuring apparatus eg 0.05 cm3 on a burette

Keep it simple!

The errors referred to are estimates –don‘t start to worry about the accuracy of the error estimates

Be aware of the various sources of error and work with the largest

Percentage Error

Recipe for Porridge:1 cup oats3.5 cups milk1 pinch salt

Percentage errorsOats 2%Milk less than 1%Salt 500%

Combining Errors 1

NaOH + HCl NaCl + H20

Imagine you try to find the molar concentration of the HCl by weighing out some NaOH, dissolving it and then doing a titration.

Sources of error:Balance error (mass of NaOH)Volume error when making up the solutionVolume error (vol of HCl used in titration)

These three sources of error clearly contribute to some overall error.

Combining Errors 2

Sample results:

Balance: 0.05g in 2.5gVolumetric Flask: 0.5 ml in 250mlBurette: 0.05ml out in 18.0ml

Percentage errors:

Balance : (0.05*100)/2.5 = 2%

Volumetric Flask: (0.5*100)/250 = 0.2%

Burette: (0.05*100)/18=0.28%

Combining Errors 3

error % error

Balance : 0.05g 2%

Volumetric Flask: 0.5ml 0.2%

Burette: 0.05ml 0.28%

Total ? 2.48%

Overall apparatus error = 2.5%

Try your hand at these

On the worksheet you printed at the start of the slideshow work out the percentage error for each piece of apparatus and then the overall apparatus error.

You may want to look back at slide 8 if you get stuck.

Interpreting experimental results 1

Relative Atomic Mass of Group II metal = 24.8

Overall apparatus error = 2.4%

The actual error = 24.8 2.4%

= 24.8 (24.8*2.4/100)

= 24.8 0.6

Or 24.2 – 25.4

RAM (Mg) = 24.3

Interpreting experimental results 2

Indigestion tablet : 500mg Calcium Carbonate

Results of your titration

Mass of Calcium Carbonate present = 483mg

Overall apparatus error = 1.8%

Find range of possible values:

483*1.8/100 = 8.7mg

= 483 8.7 mg

= 474.3 - 491.7 mg