Uncertainty
You should become familiar with the principles of Accuracy and Precision, and the corresponding principles of uncertainty and error.
What is the difference between accuracy and precision? Perhaps this diagram can help.
In experimental work you may need to calculate using uncertainties. See this video for an introduction.
Mass of beaker = 101.400g
Mass of beaker plus substance = 101.601g
For each measurement the uncertainty would be +-0.0005 (half the smallest interval).
The mass of the object is 101.601-101.400 = 0.201g
The total uncertainty in mass of the object is 0.005 + 0.005 = +- 0.001g.
So we quote our answer as 0.201 +- 0.001
This expected value (0.2) is within the error range of this measurement (0.200 to 0.202) so it's completely accurate.
If the substance was dissolved in a measured 250mL of solution with a quoted uncertainty of +- 1.0mL, then the calculation of concentration in g/L would involve a division (Mass/Vol). This requires percentage uncertainties.
The percentage uncertainty in the mass is
0.001/0.201 x 100 = 0.498%
The percentage uncertainty in the volume is
1/250 x 100 = 0.40%
The concentration (g/L) is 0.201/0.250 = 0.804 g/L
The total percentage uncertainty in the answer is 0.498% + 0.40% = 0.90%
But 0.90% of 0.804 is 0.007 g/L
So our answer is
0.804 g/L +- 0.90%
or 0.804 g/L +- 0.007 g/L
You will sometimes hear the term "Error" used when referring to uncertainty but this really refers to the variation between your answer and the accepted or expected answer.
Instructions on how to calculate the error when measuring the liquids used in the experiment. Instruction are taken off the Chemistry Class wikispaces.
You should become familiar with the principles of Accuracy and Precision, and the corresponding principles of uncertainty and error.
What is the difference between accuracy and precision? Perhaps this diagram can help.
In experimental work you may need to calculate using uncertainties. See this video for an introduction.
- When adding or subtracting measurements you add uncertainties
- When multiplying or dividing measurements you add percentage uncertainties
Mass of beaker = 101.400g
Mass of beaker plus substance = 101.601g
For each measurement the uncertainty would be +-0.0005 (half the smallest interval).
The mass of the object is 101.601-101.400 = 0.201g
The total uncertainty in mass of the object is 0.005 + 0.005 = +- 0.001g.
So we quote our answer as 0.201 +- 0.001
This expected value (0.2) is within the error range of this measurement (0.200 to 0.202) so it's completely accurate.
If the substance was dissolved in a measured 250mL of solution with a quoted uncertainty of +- 1.0mL, then the calculation of concentration in g/L would involve a division (Mass/Vol). This requires percentage uncertainties.
The percentage uncertainty in the mass is
0.001/0.201 x 100 = 0.498%
The percentage uncertainty in the volume is
1/250 x 100 = 0.40%
The concentration (g/L) is 0.201/0.250 = 0.804 g/L
The total percentage uncertainty in the answer is 0.498% + 0.40% = 0.90%
But 0.90% of 0.804 is 0.007 g/L
So our answer is
0.804 g/L +- 0.90%
or 0.804 g/L +- 0.007 g/L
You will sometimes hear the term "Error" used when referring to uncertainty but this really refers to the variation between your answer and the accepted or expected answer.
Instructions on how to calculate the error when measuring the liquids used in the experiment. Instruction are taken off the Chemistry Class wikispaces.
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