INTRODUCTION The K2[Cu(C2O4)2(H2O)2] is not soluble in water. When K2[Cu(C2O4)2(

Question

INTRODUCTION

The K2[Cu(C2O4)2(H2O)2] is not soluble in water. When K2[Cu(C2O4)2(H2O)2] is mixed with water, it decomposes and yields the oxalate ion and a precipitate of copper(II) oxalate according to the following chemical equation:

K 2[Cu(C2O4)2(H2O)2] (s) 2 K+ (aq) + CuC2O4 (s) + 2 H2O + C2O42– (aq) rxn 1 Consequently, we cannot use an aqueous solution of it to measure its absorbance although K2[Cu(C2O4)2(H2O)2] complex

is colored. In order to be able to use the spectrophotometry and determine the amount of Cu (II) ions in the synthesized complex, we need to ensure that all Cu (II) ions remain in the solution and none precipitates. This is achieved by reacting

the synthesized K2[Cu(C2O4)2(H2O)2] complex with 1.0 M NH3, so the soluble Cu(NH3)42+ colored complex is formed based on the reaction:

K 2[Cu(C2O4)2(H2O)2] (s) + 4 NH3 (aq) 2 K+ (aq) + Cu(NH3)42+ (aq) + 2 H2O + 2 C2O42– (aq) rxn 2 The rxn 2 yields a stable solution that can easily be measured spectrophotometrically to determine its copper content. Then, based on the molar ratio between the Cu(NH3)42+ complex and original K2[Cu(C2O4)2(H2O)2], the amount of Cu (II) in the synthesized K2[Cu(C2O4)2(H2O)2] can be calculated.

Since the analyte that is measured here is the Cu(NH3)42+ complex, we need to prepare a series of standard solutions of the Cu(NH3)42+ complex to create the standard curve. These solutions are prepared easily by diluting a stock solution of Cu(NO3)2 with 1.0 M NH3 according to the following reaction:

Cu(NO3)2 (aq) + 4NH3 (aq) Cu(NH3)42+ (aq) + 2NO3- (aq) rxn 3



WHAT IS THE MOLAR RATIO BETWEEN K2[Cu(C2O4)2(H2O)2] and Cu(NH3)42+   in rxn 2?

Explanation / Answer

reaction 2 is

K2[Cu(C2O4)2(H2O)2] (s) + 4 NH3 (aq) ........> 2 K+ (aq) + [Cu(NH3)4]2+ (aq) + 2 H2O + 2 C2O42– (aq)

in this reaction one mole K2[Cu(C2O4)2(H2O)2] react with 4 mole NH3 to form one mole [Cu(NH3)4]2+.

so molar ratio between K2[Cu(C2O4)2(H2O)2] and [Cu(NH3)4]2+ is 1 : 1.

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