2. 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: worksheetworksheet

3. Sources of Error 1 Reading the scale: Usually a scale can be read to  0.5 division

4. 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

5. 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 cm 3 on a burette

6. 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

7. Percentage Error Recipe for Porridge: 1 cup oats 3.5 cups milk 1 pinch salt Percentage errors Oats  2% Milk less than 1% Salt  500%

8. Combining Errors 1 NaOH + HCl NaCl + H 2 0 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 solution Volume error (vol of HCl used in titration) These three sources of error clearly contribute to some overall error.

9. Combining Errors 2 Sample results: Balance:  0.05g in 2.5g Volumetric Flask:  0.5 ml in 250ml Burette:  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%

10. Combining Errors 3 error% error Balance : 0.05g 2% Volumetric Flask: 0.5ml 0.2% Burette: 0.05ml0.28% Total?2.48% Overall apparatus error = 2.5%

11. 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.

12. 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

13. 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