SOLUTION PREPARATION AND BEER’S LAW ADDITIONAL READING The concepts in this experiment are also discussed in sections 4.5, 4.9, 6.1– 6.2, 13.1 of Chemistry and Chemical Reactivity by Kotz, Treichel, Townsend and Treichel, and in section 4.5, 6.1, 6.2, 13.1, and 23.4c of Mindtap General Chemistry by Vining, Young, Day, and Botch A history of Kool-Aid ® can be found at: (checked 04-Jun-2018). ABSTRACT This is a two-part experiment. In part A, you and your lab partner will prepare solutions of your assigned Kool-Aid ® flavor(s). In part B, those solutions are used to construct standard Beer’s Law plots of absorbance vs. concentration. The best-fit lines for those plots are then used to determine dye concentrations in Kool-Aid ® of known and unknown dilutions. Background material includes topics not covered in your lecture text. The experiment employs the Vernier colorimeter. SOLUTION PREPARATION For quantitative work we need to know the amount of solute in solutions. Concentration is most commonly measured as molarity ( M ), which is defined as: ( 1 ) Molarity is a measure of the number of solute molecules present per liter of solution (NOT SOLVENT). P REPARATION OF A SOLUTION USING A PURE , SOLID SOLUTE For 3-4 significant figure accuracy, weighed, dry solute is dissolved in less than the required volume of solution in a volumetric flask (choose Class A for highest accuracy). Once dissolution (and adjusting to near the glassware’s calibration temperature, typically 20°C) is complete, add solvent, and mix again. Add more solvent with a transfer pipet to bring the solution level up to the fill line on the flask’s neck. The bottom center of the liquid’s meniscus should be level with the top of the line. Stopper or cap the flask; hold the top in place and support the base. Repeatedly invert the flask gently, swirl it, and revert it until mixing is essentially complete (up to 10 times, at least a few times beyond when inhomogeneity – streaking – is still visible). How should 250 mL of 0.200 M (mol/L) CuSO 4 solution be prepared, given a 500 g bottle of copper(II) sulfate pentahydrate CuSO 4 · 5H 2 O (beautiful blue crystals) and deionized (DI) water? We need the molar mass of CuSO 4 · 5H 2 O. copper 1 x 63.546 g/mol sulfur 1 x 32.065 g/mol oxygen 9 x 15.999 g/mol (4 from sulfate, 5 from 5H 2 O) hydrogen 10 x 1.0079 g/mol (from the 5 waters of hydration) copper(II) sulfate pentahydrate 249.681 g/mol We can now use the molar mass to calculate the mass of solute required: 249.681 g/mole x .200 mole/L x 0.250 L = 12.5 g CuSO 4 · 5H 2 O A top-loading balance is sufficient here. Use a small (50-100 mL) beaker as the weighing vessel and a powder funnel to help quantitatively transfer the weighed mass to a 250 mL volumetric flask (Class A: 250.00±0.12 mL). Dissolve and rinse-in any adhering crystals with a DI-water squirt bottle. Using a beaker (tolerance typically ±5%) to measure volume wouldn’t be accurate n sol' L n M or solution of Liters solute of moles Molarity = =
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- Spring '06