Pipets used for the transfer of samples of aqueous solutions are always rinsed w
ID: 478557 • Letter: P
Question
Pipets used for the transfer of samples of aqueous solutions are always rinsed with a small portion of the solution to be used before the sample is taken. Calculate the percentage error arising in an experiment if 1-mL, 5-mL, and 10-mL pipets are used for transfer and each pipet contains 5 drops of water adhering to the inside of the barrel. A single drop of water has an approximate volume of 0.05 mL. It is important to make certain that there is no air bubble in the tip of the beret below the stopcock before the initial reading of the liquid level in the burette is taken. If a 0.5-mL air bubble is present in the tip of a burette, what percent error in 10-mL, 20-mL, and 40-mL samples will result if the air bubble is dislodged during the dispensing of the samples?Explanation / Answer
1.
% Error = (Measured Value – Actual Value) / Actual Value * 100
Volume of 1 drop of water = 0.05 mL
Volume of 5 drops of water = 5 * 0.05 = 0.25 mL
Measured Value – Actual Value = Volume of 5 drops of water = 0.25 mL
a) 1 mL pipet
Actual Value = 1 – 0.25 = 0.75 mL
% error = 0.25 / 0.75 * 100 = 33.33 %
b) 5 mL pipet
Actual Value = 5 – 0.25 = 4.75 mL
% error = 0.25 / 4.75 * 100 = 5.26 %
c) 10 mL pipet
Actual Value = 10 – 0.25 = 9.75 mL
% error = 0.25 / 9.75 * 100 = 2.56 %
2.
Volume of air bubble present = 0.5 mL
Measured Value – Actual Value = Volume of air bubble present = 0.5 mL
a) 10 mL buret
Actual Value = 10 – 0.5 = 9.5 mL
% error = 0.5 / 9.5 * 100 = 5.26 %
b) 20 mL buret
Actual Value = 20 – 0.5 = 19.5 mL
% error = 0.5 / 19.5 * 100 = 2.56 %
c) 40 mL buret
Actual Value = 40 – 0.5 = 39.5 mL
% error = 0.5 / 39.5 * 100 = 1.27 %