Preparing a solution of a known concentration is an essential skill to scientists. Included in this skill is the ability to make dilute solutions from more concentrated solutions that have been prepared previously. The concentration of the new, more dilute solution can be calculated using:
CConc. Solution x VConc. Solution = CDil. Solution x VDil. Solution
(OR)
CCon x VCon =CDil x VDil
where "C" stands for concentration and "V" stands for volume.
One problem with making a dilute solution from a much more concentrated solution is that the required volume of the concentrated solution is often very small and almost impossible to measure accurately. For example, suppose you were asked to prepare 100.0 mL of a solution containing exactly 0.50 ppm Zn2+ from a 1000.0 ppm solution (the "ppm" unit, like molarity, is a measure of concentration and stands for "parts-per-million"; for example, a 1 ppm Zn2+ solution contains exactly 1 gram of zinc per million grams of solution). Using the equation above, this would mean that you would need to transfer:
CCon
x VCon =CDil
x VDil
VCon = CDil
x VDil/CCon
VCon = (0.50 ppm) x (100 mL)/
1000
ppm
VCon = 0.050 mL
which corresponds to about one drop of the concentrated solution. Since this volume is almost impossible to measure accurately, scientists have found a better way to do this using "serial dilution." Serial dilution is a method where a solution of intermediate concentration is prepared from the more concentrated solution. This intermediate solution is then used to prepare the final dilute solution.
This event is designed to test your ability to accurately prepare a dilute solution from a more concentrated solution using serial dilution. At the beginning of the event, each team will be asked to prepare a solution of Zn2+ with a concentration between 0.50-1.50 ppm. To complete this task, each group will be provided with appropriate equipment (1-mL and 10-mL variable volume Mohr pipets; 50-mL, 100-mL, & 250-mL volumetric flasks; pipet bulb; distilled water; etc.) and a concentrated solution of about 1000.0 ppm Zn2+. Each group has 30 minutes to prepare the assigned concentration of Zn2+ using serial dilution. The concentration and volume of the intermediate solution are at the discretion of each team, but should be selected so as to permit preparation of the dilute solution with the supplied glassware. Once the dilute solution has been prepared, it should be submitted with your calculations to the Field Day Staff for evaluation.
Points will be awarded as follows: the percent error will be subtracted from 100 to create you raw score. For example, if your team was to prepare a 1.00 ppm solution and this solution measured 0.84 ppm, then your relative error would be (1.00 - 0.84) x 100% = 16%. Your team then would receive 100 Ñ 16 = 84 points for the event. If your team submits the solution prior to the end of the thirty minute period, one extra point will be added to your score for each minute early. The Field Day Staff will deduct points for all safety violations, including not wearing goggles or using the pipets in a manner other than that demonstrated. |
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