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Chapter 12: Q3 TY (page 275)
Find if free, unprotonated [NH3] = 0.02 M.
=0.00673 if free, unprotonated [NH3] = 0.02 M.
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Indirect EDTA determination of cesium. Cesium ion does not form a strong EDTA complex, but it can be analyzed by adding a known excess of NaBiI4 in cold concentrated acetic acid containing excess NaI. Solid Cs3Bi2I9 is precipitated, filtered, and removed. The excess yellow is then titrated with EDTA. The end point occurs when the yellow color disappears. (Sodium thiosulfate is used in the reaction to prevent the liberated from being oxidized to yellow aqueous I2 by O2 from the air.) The precipitation is fairly selective for Cs+. The ions Li+, Na+, K+, and low concentrations of Rb+ do not interfere, although Tl+ does. Suppose that 25.00 mL of unknown containing Cs+ were treated with 25.00 mL of 0.08640 M NaBiI4 and the unreacted required 14.24 mL of 0.0437 M EDTA for complete titration. Find the concentration of Cs+ in the unknown.
Spreadsheet equation for auxiliary complexing agent. Consider the titration of metal M (initial concentration = CM, initial volume = VM) with EDTA (concentration = CEDTA, volume added = VEDTA) in the presence of an auxiliary complexing ligand (such as ammonia). Follow the derivation in Section 12-4 to show that the master equation for the titration is
where is the conditional formation constant in the presence of auxiliary complexing agent at the fixed pH of the titration (Equation 12-18) and [M]free is the total concentration of metal not bound to EDTA. [M]free is the same as [M] in Equation 12-15. The result is equivalent to Equation 12-11, with [M] replaced by [M]free andreplaced by .
The sulfur content of insoluble sulfides that do not readily dissolve in acid can be measured by oxidation with Br2 to .25 Metal ions are then replaced with H+ by an ion-exchange column, and sulfate is precipitated as BaSO4 with a known excess of BaCl2. The excess Ba2+ is then titrated with EDTA to determine how much was present. (To make the indicator end point clearer, a small, known quantity of Zn2+ also is added. The EDTA titrates both the Ba2+ and the Zn2+.) Knowing the excess Ba2+, we can calculate how much sulfur was in the original material. To analyze the mineral sphalerite (ZnS, FM 97.46), 5.89 mg of powdered solid were suspended in a mixture of CCl4 and H2O containing 1.5 mmol Br2. After 1 h at 200 C and 2 h at 500 C, the powder dissolved and the solvent and excess Br2 were removed by heating. The residue was dissolved in 3 mL of water and passed through an ion-exchange column to replace Zn2+ with H+. Then 5.000 mL of 0.014 63 M BaCl2 were added to precipitate all sulfate as BaSO4. After the addition of 1.000 mL of 0.010 00 M ZnCl2 and 3 mL of ammonia buffer, pH 10, the excess Ba2+ and Zn2+ required 2.39 mL of 0.009 63 M EDTA to reach the Calmagite end point. Find the weight percent of sulfur in the sphalerite. What is the theoretical value?
Calculate [HY3-] in a solution prepared by mixing 10.00 mL of 0.010 0 M VOSO4, 9.90 mL of 0.010 0 M EDTA, and 10.0 mL of buffer with a pH of 4.00
Consider the titration of 25.0 mL of 0.020 0 M MnSO4 with 0.010 0 M EDTA in a solution buffered to pH 8.00. Calculate pMn2+ at the following volumes of added EDTA and sketch the titration curve:
(a) 0 mL (b) 20.0 mL (c) 40.0 mL (d) 49.0 mL (e) 49.9 mL (f) 50.0 mL (g) 50.1 mL
(h) 55.0 mL (i) 60.0 mL
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