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Chapter 12: Q8P-c (page 283)

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

Short Answer

Expert verified

(c) For 40 mL the value ofpMn2+is 2.81.

Step by step solution

01

Introduction

Equations and data obtained in order to proceed for calculation are as follows

Titration  Reaction: Mn2++EDTA⇌MnY2−Kf=1013.89At   pH   8  αY4−=4.2×10−3 Table 12−1

02

Determine equilibrium constant

Kf'=αY4−×Kf=4.2×10−3×1013.89=3.3×1011

Equivalence point=50 mL

03

Determine the value of pMn2+

The concentration of the remaining product can be calculated using the following equation

=Fraction remaining × Initial concentration × Dilution factor

If 40 mL solution is added then the reaction will be 40/50 completed as the equivalence point is at 50 mL. Then the metal (Mn) concentration will be

Mn2+=50−40500.02 M2525+40=1.54×10−3 M

Therefore, the value of pMn2+

pMn2+=−logMn2+=−log1.54×10−3=2.81

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Most popular questions from this chapter

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?

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

ϕ=CEDTAVEDTACMVM=1+Kf′′[M]free−[M]free+Kf′′[M]freeCMKf′′[M]free+[M]free+Kf′′[M]free2CEDTA

where Kf''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 andKfreplaced by Kf''.

Pyrocatechol violet(Table 12-3) is to be used as a metal ion indicator in an EDTA titration. The procedure is as follows:

1. Add a known excess of EDTA to the unknown metal ion.

2. Adjust the pH with a suitable buffer.

3. Back-titrate the excess chelate with standard Al3+.

From the following available buffers, select the best buffer, and then state what color change will be observed at the end point. Explain your answer.

  1. pH 6–7 (ii) pH 7–8 (iii) pH 8–9 (iv) pH 9–10

Calcium ion was titrated with EDTA at pH 11, using Calmagite as indicator (Table 12-3). Which is the principal species of Calmagite at pH 11? What color was observed before the equivalence point? After the equivalence point?

Find [Ca2+] in 0.10 M CaY2- at pH 8.00
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