An ordinary laboratory pipette may be expected to deliver its nominal volume with good precision and good accuracy if it is used in the way recommended. In this experiment we investigate the precision and accuracy of such a pipette by making accurate determinations of the mass of water it delivers in repeated operations.
10.0-cm3 pipette, 50-cm3 beaker, thermometer, pipette filler, graph paper.
Procedure (Read carefully)
1.Clean the beaker and the pipette and dry the beaker.
2.Obtained distilled water in an Erlenmeyer flask and let it stand on the bench for about 15 min before determining its temperature.
3.Weigh the beaker on a balance which allows you to determine the mass to the nearest tenth of a milligram (i.e. ± 0.001 g).
4.Fill (use the pipette filler) and discharge the pipette as recommended, into the beaker, and determine the mass of water discharged by taking the difference.
5.Repeat step 4 until you have the results of four (4) such trials.
Note: You do not need to empty and dry the beaker between trials. Also, you should try not to spend more than a half-hour in the balance room doing this, so that everyone can have a chance to use the balances in the time allotted.
7.Determine the temperature of the water you have pipetted and take the mean of the two temperatures you have measured as the effective temperature of the water during the calibration.
1. Find the mean of the four trials. The mean volume can now be determined from the mean mass and the density of water at the temperature you determined in 6 above. However, there is a correction to be made for the upthrust of air on the water during the weighing. In normal day-to-day use of balances we neglect this correction, but now that we are interested in accurate calibration, we will take this into account. The correction occurs because the upthrust of air on...