Question

EQUILIBRIUM OF A METAL COMPLEX ION Part 2: Determination of the Equilibrium Constant 1. Write the balanced chemical equation for the reaction we will be studying in this experiment and its associated equilibrium constant expression. 2. What analytical technique will we be using to measure concentrations? Which species can we measure with this technique? State Beer's Law and define its terms. 3. 4. 5. The following data were collected by a student preparing standards for a Beer's Law plot Discuss the purpose of the Beer's Law plot. of compound X. The solutions were made by diluting a stock solution that was 0.250 M in X. Solution 1 was prepared by pipetting 1.12 mL of solution X and 3.87 mL of water as shown in Table 2 and mixing. (Assume that the volumes are additive.) Table 2: Beer's Law Data for "X" Solution # 1 Volume(mL) of 0.250 MX 1.12 | | Volume (mL) 3.87 3.06 2.16 0.78 0 Abs 0.211 0.380 0.550 0.780 0.940 2.09 3.06 3.97 4.92 Determine the concentration of X in each of the 5 resulting solutions. Make a table showing the calculated concentration and the measured absorbance. Use this table to create a Beer's Law plot (on Excel) with a linear trendline fit (select the option of forcing the trendline have a 0 intercept). Determine the value of Beer's constant for X at this wavelength. What would be the [X] for a solution whose absorbance was 0.475? 6. For our experiment in lab, creating a standard absorbance curve will require solutions of known concentrations of Fe(SCN)", which will be prepared by mixing known amounts of Fe with a large excess of SCN and assuming that the reaction goes "essentially to completion". Describe a way that you could experimentally test the validity of the assumption that the reaction has gone to completion.

Answer 1

Fe^3+_(aq) + SCN^-_(aq) [Fe (SCN)]^2+_(aq) k_eq = [[Fe (SCN)]^2+]]/[Fe^3+] [SCN^-] \r\n Colourimetry The species should be a colored compound. (abs between visible region) A = sigma Cl A = absorbance sigma = molar absorptivity C = concentration l = length of path travelled by light \r\n Beer's la is use to measure concentration of unknown solutions using a standard calibration plot. If we have a solution of 'X' which is an unknown solution. we want to find the concentration of that. We can prepare a concentration series of known X and measure absorbance and plot the calibration curve of Absorbance vs concentration. Then unknown XS' absorbance will be measured and using the calibration curve can be read the concentration of unknown X relevant to it's absorbance. To Find concentrations of each solution use following equation C_1 V_1 = C_2 V_2 -----(1) C_1 = initial concentration V_1 = volume taken