Chemistry Laboratory
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Working in a chemistry laboratory where mixing chemicals is crucial to scientific advancement, scientists have developed techniques involving mixing and measuring chemical solutions. To do so, scientists first need to create a pure and accurately measured standard solution. Scientists create this solution by dissolving a primary standard into a measured volume of solvent. A primary standard is a pure substance with a high weight such that the instrumentation error is negligible.
Scientists have also developed laboratory techniques to find concentrations of unknown solutions. To explain their approach, we need to explore the properties of light and its interaction with molecules in a solution. Light is a result of electrons moving between energy levels. When electrons go from a high to low energy level, photons are emitted; conversely, when photons interact with atoms, absorption may occur. Beers Law is a direct result of this phenomenon, stating that light absorption in a solution is directly proportional to the concentration of the solution. The relation is as follows: Absorbance = Molar Absorbtivity * Path Length * Concentration.
For Beers law to hold, the absorbance needs to be between .1A and 1A. Plugging these values for “Absorbance” into the equation, the student can find the concentration range that will work under Beers Law.
Using one pipet during each step, the student has 6 solutions he can create: 1) 5 mL into a 50 mL flask, 2) 5 mL into a 100 mL flask, 3) 10 mL into a 50 mL flask, 4) 10 mL into a 100 mL flask, 5) Solution # 1 into a 100 mL flash, 6) Solution #3 into a 100 mL flask.
Using titration calculations given initial concentration and initial and final volumes, new concentrations can be found and compared to the range of accepted concentrations. Solutions 1, 3, 4 and 6 fit this criterion.
These solutions can then be entered into Beers Law to find absorbance. The results are:
Solution 1, 3, & 5: .136A
Solution 3: .272A
The absorbance measurements are limited in significant figures by the instrument with the least precision, which in this case is the 5.00 mL pipet with three significant figures. In conclusion, Beers law has helped scientists more easily mix chemicals accurately in the laboratory.