Uncertainty & error questions

Assignment: Uncertainty & error short answer questions

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Explain why repeating a measurement and taking the average of five separate readings of data will decrease the random error but have no effect on the systematic error.

Words: 1-100
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A balance has been wrongly calibrated so that it always records a reading that is 1.00 g too high.
Explain how, without recalibrating it, the balance could be used to obtain:

(a) an inaccurate and precise result for the mass of a piece of magnesium metal.

Words: 1-100
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(b) an accurate and precise result for the mass of a piece of magnesium metal.

Words: 1-100
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A 100 cm3 measuring cylinder has an uncertainty of ± 1.00 cm3. A student used the measuring cylinder to measure 25.00 cm3 of a solution into a burette. The burette has an uncertainty of 0.05 cm3. Assuming no other errors what range of readings could the burette give for the accurate transfer of the 25.00 cm3 of solution.

3 lines
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A stop-watch states that it can record to ± 0.01 s. A student used this stop-watch to record measurements every 30 seconds for the volume of gas evolved in a particular reaction. She recorded her values of time with an uncertainty of ± 1 s. Explain why she gave the uncertainty as ± 1 s rather than ± 0.01 s.

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1.541 ± 0.001 g of hydrated oxalic acid crystals were dissolved in distilled water. The solution was placed in a 250.0 ± 0.5 cm3 volumetric flask and the total volume made up to the mark with distilled water. After thoroughly mixing the contents, 25.00 ± 0.04 cm3 of this solution was pipetted into a conical flask. This required 24.90 ± 0.08 cm3 of 0.100 ± 0.001 mol dm3 sodium hydroxide solution to be neutralised completely.
The equation for the reaction is:
                     (COOH)2(aq)  +  2NaOH(aq) →  (COONa)2(aq)  +  2H2O(l)

(a) Calculate the percentage uncertainty associated with:
(i) the mass of the oxalic acid

2 lines
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(ii)  the volume of the solution in the volumetric flask.

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(iii) the volume in the pipette.

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(iv) the molarity of the sodium hydroxide solution.

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(v)  the volume of sodium hydroxide solution used.

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(b) Calculate the total percentage uncertainty for this experiment.

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(c) Calculate the molar mass of the oxalic acid obtained from these experimental results to the correct number of significant figures and state the uncertainty.

10 lines
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(d) The correct literature value for the molar mass of the acid is 126.06 g mol-1. Calculate the percentage error.

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(e) State any other assumptions which may have affected the accuracy of the result.

3 lines
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