A solution containing Pb 2+ was found to have a concentration of 0.0552 (+/- 0.0

Question

A solution containing Pb2+ was found to have a concentration of 0.0552 (+/- 0.0001) M by percipitating the ion as lead sulfide and measuring the resulting mass. A 5mL portion of this solution was collected using an appropirate volumetric pipet and placed in a 100mL volumetric flask which was then brought to volume using distilled water.

(a) Determine the concentration of Pb2+ in parts per million (mg/L) that was diluted sample

(b) Calculate the absolute uncertainty of the concentration of Pb2+ in the diluted sample. Assume the molar mass of lead has an absolute uncertainty of +/- 0.1 g/mol

(c) Give two specific sources of error the could occur during the process describe above and explian how it would affect the final results

Explanation / Answer

a) Concentration of Pb2+ after dilution:

M1 = 0.0552 M, V1 = 5 ml

M2 = ?, V2 = 100 ml

M1V1 = M2V2

M2 = M1V1/V2 = 0.0552*5/100 = 0.00276 M

In parts per million: 0.00276 M = 0.00276 g/L = 2.76 mg/L

b) Absolute uncertainity: If M1 = 0.0001 M

Then M2 = 0.0001*5/100 = 5*10-6 M = 5*10-3 mg/L

Absolute uncertainity = +/- 0.005

c) The specific sources of error in the experiment may be:

(i) The volumetric pipet may contain bubbles of air while filling it up, then the volume taken would be less than 5 ml and thus the final volume will have less concentration than the actual one.

(ii) The student might take upper miniscus in volumetric flask while filling it up to 100 ml. Then the total volume would be less than 100 ml. The final concentration would be less then the desired concentration.

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