Percentage Measurement Uncertainty

Question 1

what does the magnitude of measurement uncertainties for an instrument depend on

Answer 1

• they way its been manufactured

- they way it is calibrated

Question 2

what is the difference between the measurement uncertainty of an instrument and the percentage uncertainty

Answer 2

• the measurement uncertainty is usually fixed

- whereas the percentage uncertainty changes depending on the value you measure

Question 3

what two variables change the percentage uncertainty of an instrument

Answer 3

• the number of values you have recorded that are related to each other
• how big the value is compared to the capacity of the apparatus

Question 4

what is the common measurement uncertainty of a 50cm^3 burette

Answer 4

+-0.05cm3

Question 5

a titre measured with a burette is 22.5cm3. what would the measurement uncertainty of the burette be now

Answer 5

• +-0.1cm3
• because you have measured the reading on the burette twice (initial and final)
• meaning you double the uncertainty of 0.05cm3

Question 6

how would the percentage uncertainty generally be calculated

Answer 6

• divide the uncertainty by the value you have recorded

- then multiply by 100 to get the percentage

Question 7

what would the percentage uncertainty of the burette be

Answer 7

(0.1 / 22.5) x 100 = +-0.44%

Question 8

what does the percentage uncertainty using a balance depend on

Answer 8

• the precision of the balance

- the mass being weighed

Question 9

what would increasing both variables each do to the percentage uncertainty

Answer 9

• increasing the precision would lead to the percentage uncertainty decreasing
• increasing the mass being weighed would also decrease the percentage uncertainty

Question 10

if you were measuring the mass of a 3.57g marble chip with a balance that has an uncertainty of +-0.005g, what would the percentage uncertainty be

Answer 10

• 2 x 0.005 = 0.01

- (0.01 / 3.57) x 100 = +-0.28%

Question 11

why do you multiply the measurement uncertainty by 2 when you are only taking one measurement

Answer 11

• there is a measurement uncertainty when calibrating the balance
• as well as a measurement uncertainty when measuring the mass
• so you need to double it

Question 12

if you were using multiple instruments to obtain a value, how would you calculate the total measurement uncertainty for that value if nothing was squared or cubed

Answer 12

• you would work out the percentage uncertainty for each instrument you used
• then you would add them up to get the total percentage uncertainty
• you would then divide your main value by 100 and multiply that by your total percentage uncertainty
• giving you the measurement uncertainty

Question 13

if your main final value was 100cm3 and your total percentage uncertainty was +-5%, what would the measurement uncertainty be

Answer 13

(100cm3 / 100%) x 5% = +-5cm3

Question 14

how could you minimise the error and uncertainty in a thermochemistry experiment

Answer 14

• minimising heat energy losses with insulation

- using a more accurate balance wouldnt have much of an effect

Question 15

how could you minimise the error and uncertainty of experiments in general

Answer 15

• use higher values (volumes / masses) rather than lower ones
• this will mean that the division between the uncertainty and the value would result in a much smaller value
• so when multiplied by 100 it would be a lot smaller as well