Practice Osukuuni Questions
CHEM 3811
EDTA
1. Briefly explain water hardness. What is the difference between temporary and permanent hardness. Refer notes 2. Outline the experimental procedure for each of the following EDTA titrations and state the circumstances that might make them necessary. (a) direct titration (b) back titration (c) indirect titration (d) displacement titration Refer notes 3. What are the purposes of the use of auxiliary complexing agent and masking agent? Differentiate between the two types of agents. Give two examples each. Refer notes 4. A 22.00 mL sample containing Fe3+ was treated with 9.50 mL of 0.0325 M EDTA to form a complex. Excess EDTA was left in the solution. The excess EDTA was back titrated with Mg2+. It took 2.15 mL of 0.0425 M Mg2+ to reach the end point. Determine the concentration of Fe3+ in the original solution. EDTA forms 1:1 complexes Moles EDTA bound to Fe3+ = moles Fe3+ in sample Moles EDTA added to sample = (0.00950 L)(0.0325 M) = 3.09 x 10-4 mol Moles excess EDTA = moles Mg2+ = (0.00215 L)(0.0425 M) = 9.14 x 10-5 mol Moles EDTA bound to Fe3+ = 3.09 x 10-4 mol - 9.14 x 10-5 mol = 2.18 x 10-4 mol Molarity of Fe3+ = 2.18 x 10-4 mol/0.02200 L = 9.91 x 10-3 M 5. A 50.00 mL aliquot of a solution containing iron(II) and iron(III) required 13.73 mL 0.01200 of M EDTA when titrated at pH 2.0 and 29.62 mL when titrated at pH 6.0. Express the concentration of the solution in terms of the parts per million of each solute. Trivalent cations form more stable complexes at pH around 1 than divalent cations At very low pH, divalent cations form unstable complexes Additional information: Fe3+ is more acidic than Fe2+ pKa of Fe3+ = 2.18 and pKa of Fe2+ = 9.39 At pH 2.0, only Fe3+ stable complexes may form At pH 6.0, both Fe2+ and Fe3+ complexes may form At pH = 2.0 Moles Fe3+ = moles EDTA = (0.01373 L)(0.01200 M) = 1.648 x 10-4 mol Concentration Fe3+ = 1.648 x 10-4 mol/0.05000 L = 3.295 x 10-3 mol/L = 184.0 mg/L
At pH = 6.0 Moles Fe3+ + Fe2+ = moles EDTA = (0.02962 L)(0.01200 M) = 3.554 x 10-4 mol Moles Fe2+ = 3.554 x 10-4 mol - 1.648 x 10-4 mol = 1.906 x 10-4 mol Concentration Fe2+ = 1.906 x 10-4 mol/0.05000 L = 3.812 x 10-3 mol/L = 212.9 mg/L 6. A 31.69 mL aliquot of 0.01470 M Mg2+ solution was titrated with an EDTA solution. The initial and final readings of the buret containing the EDTA were 0.04 mL and 46.39 mL, respectively. What is the concentration of the EDTA solution? Moles EDTA = moles Mg2+ = (0.03169 L)(0.01470 M) = 4.658 x 10-4 mol Volume EDTA = 46.39 - 0.04 = 46.35 mL Molarity EDTA = 4.658 x 10-4 mol/0.04635 L = 0.01005 M EDTA