|Precision |The precision of each of the measurements you made is indicated by the % uncertainty calculated. | |Accuracy |The accuracy of an overall result (concentration of acid) means how close it is to the ‘true’ value. | | |It is possible to make very precise measurements but still end up with an inaccurate result because of a weakness | | |in the procedure. | |Reliable |You repeat measurements and if they are close then they are said to be reliable. A single measurement could be an | | |outlier. | |Limitation |This is a stage in the procedure which could have led to an inaccuracy in the overall result (concentration of | | |acid). |
A little book of tips for titrations.
• Recording results
• Calculating the average titre
• Evaluation of results
• Evaluation of procedures
Recording results and calculating the average/mean titre:
|Titration |Rough |1 |2 |3 |4 |5 | |Initial burette reading/cm3 |0.00 |22.00 |1.00 | | | |
Average titre = 21.85 + 21.95 / 2 = 21.90 (quote to 2 decimal places)
Titration |Rough |1 |2 |3 |4 |5 | |Final burette reading/cm3 |22.00 |43.85 |23.15 |44.10 | | | |Initial burette reading/cm3 |0.00 |22.00 |1.00 |22.15 | | | |Titre/cm3 |22.00 T |21.85 √ |22.15 √ |21.95 √ | | | | Average titre = 21.85 + 22.15 + 21.95 / 3 = 21.98 (there is a rule that you use any titres within a 0.3cm3 range of the concordant titres to calculate the average)
What is the average titre?
• 21.10 + 21.50 + 21.20?
Evaluation has two aspect – evaluation of measurements and evaluation of procedures (same as GCSE now)
Evaluation of results:
We look at how precise the measurements are. We look at the uncertainty associated with a measurement (sometimes called the precision error). Then we calculate the percentage uncertainty (sometimes called the percentage error).
For graduated equipment the uncertainty / precision error is taken to be half a division on either side of the smallest unit on the scale you are using. For volumetric (or ungraduated) measuring equipment you use the precision error associated with it.
Using the balance:
The uncertainty associated with a two decimal place balance reading = ±0.005g
The percentage uncertainty associated with a reading of 119.06g = 0.005 x 100 = 0.004%
The uncertainty associated with the difference between two balance readings = 2 x 0.005 = ±0.010g
The percentage uncertainty in weighing out a mass of 2.56g = 0.010 x 100 = 0.4%
Using the 250cm3 volumetric flask:
The uncertainty associated with a volumetric flask reading = ±0.2cm3
The percentage uncertainty associated with volumetric flask reading = 0.2 x 100 = 0.08%
Using the 25cm3 pipette:
The uncertainty associated with a pipette reading = ±0.06cm3
The percentage uncertainty associated with pipette reading = 0.06 x 100 = 0.2%