Question

Sulfide ion was determined by indirect titration with EDTA. To a solution containing 25.00 mL of 0.04332 M Cu(ClO₄)₂ plus 15 mL of 1 M acetate buffer (pH 4.5) were added 25.00 mL of unknown sulfide solution with vigorous stirring. The CuS precipitate was filtered and washed with hot water. Ammonia was added to the filtrate (which contained excess Cu²⁺) until the blue color of Cu(NH₃)₄²⁺ was observed. Titration of the filtrate with 0.039 27 M EDTA required 12.11 mL to reach the murexide end point. Calculate the molarity of sulfide in the unknown.

Short answer

The molarity of sulfide in the unknown will be 0.0243M

Step-by-step solution

Given Information

Amount of unknown sulfide (S^2- ) solution = 25 mL

Amount of Cu(CIO₄)₂ added = 25.0 mL of 0.04332 M Cu(CIO₄)₂

EDTA required for complete titration to reach murexide end point =12.11 mL of 0.03927 M EDTA

Determine the amount of S2- reacted

Precipitation reaction

${\mathrm{Cu}}^{2+}+{\mathrm{S}}^{2-}\to \mathrm{CuS}\left(\mathrm{s}\right)$

Total amount of Cu²⁺ used

$$\begin{gathered} =(25.00\mathrm{mL})(0.04332\mathrm{MCu}({\mathrm{CIO}}_4{)}_2) \\ =1.083\mathrm{mmol} \end{gathered}$$

Amount of EDTA reacted= Excess amount of Cu²⁺ present

Therefore, excess amount of Cu²⁺

$$\begin{gathered} =(12.11\mathrm{mL})(0.03927\mathrm{M}) \\ =0.4756\mathrm{mmol} \end{gathered}$$

Therefore, the amount of S^2- reacted

$$\begin{gathered} =(1.083-0.4756)\mathrm{mmol} \\ =0.6074\mathrm{mmol} \end{gathered}$$

Determine the concentration of sulfide in the unknown solution

The number of S^2- reacted =0.6074 mmol

Volume of unknown solution taken =25 mL

Therefore, the concentration of sulfide in the unknown will be

$$\begin{gathered} =\frac{0.6074\mathrm{mmol}}{25.00\mathrm{mL}} \\ =0.0243\mathrm{M} \end{gathered}$$

Therefore, the molarity of sulfide in the unknown will be 0.0243M