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Chapter 12: Q29P (page 285)
Give an example of the use of a masking agent
The Mg2+ in a solution of Mg2+ and Fe3+ must be titrated by EDTA. If the Fe3+ is masked with to form which does not react with EDTA.
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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 the concentration of H2Y2- at the equivalence point in Exercise 12-C
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
Spreadsheet equation for formation of the complexes ML and ML2.Consider the titration of metal M (initial concentration = CM, initial volume = VM) with ligand L (concentration = CL, volume added = VL), which can form 1:1 and 2 : 1 complexes:
Let 伪M be the fraction of metal in the form M, 伪ML be the fraction in the form ML, and be the fraction in the form ML2. Following the derivation in Section 12-5, you could show that these fractions are given by
role="math" localid="1667801924683"
The concentrations of ML and ML2are
because is the total concentration of all metal in the solution. The mass balance for ligand is
By substituting expressions for [ML] and [ML2] into the mass balance, show that the master equation for a titration of metal by ligand is
Calculate pCu2+ at each of the following points in the titration of 50.00 mL of 0.001 00 M Cu2+ with 0.00100 M EDTA at pH 11.00 in a solution with [NH3] fixed at 1.00 M:
(a) 0 mL(b) 1.00 mL (c) 45.00 mL (d) 50.00 mL (e) 55.00 mL
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