1. What is the chemical reaction that will dissolve the copper? Redox Slachim Cs
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Question
1. What is the chemical reaction that will dissolve the copper? Redox Slachim Cs) cg) Both the copper and the zine in the brass unknown will be oxidized. How is the 2. copper selectively analyzed to determine its concentration in the brass sample? 3. (a) What mass of pure copper should be used to prepare the standards? (b) What mass range of unknown sample will you Prepare an outline of the procedure on pages 3 and 4. Use one or two short weigh out if your unknown is an ore? 4. statements for each paragraph indicating the operation being performed. Alternatively, you can use pictures to diagram each step on the back of your prelab. (a) If the 0.0256 grams of pure copper wire was taken thru the procedure, what is the molarity of the copper-TEA solution in the 250 mL volumetric flask? 5. (b) If 20 mL of this solution is diluted to 100mL, what is the new molarity of copper-TEA in this solution? 6. When you set up the spectrophotometer, what solution do you use as the blank? This is the solution that allows the maximum light to be transmitted. 7. (Bonusl) In the historical story, what three substances did the nitric acid act upon?Explanation / Answer
1) Copper dissolves in nitric acid (strong oxidizing agent) according to the redox reaction equation:
Cu(s) +4HNO3 (aq) --->Cu(NO3)2 (aq)+ 2H2O(l) +2NO2(g)
Thus Cu (Oxidation state=0) is oxidized to Cu2+(Oxidation state=+2) and NO3-(Oxidation state=+5) is reduced to NO2 (Oxidation state=+4)
2)Zn is also oxidized in HNO3 when brass is treated with nitric acid:
Zn(s) +8HNO3 (aq) --->3Zn(NO3)2 (aq)+ 4H2O(l) +2NO(g)
As both Zn2+ and Cu2+ ions are present in solution, so to analyze Cu2+ content in the solution, NH3 or ammonia solution is added .Cu reacts with NH3 to form soluble colored complex [Cu(NH3)4]2+ which absorbs light in the visible region of the UV/vis electromagnetic spectrum. Thus,the concentration of Cu2+ is proportional to the concentration of complex formed ,which inturn is proportional to the Absorbance of the solution.
Absorbance,measured spectrophotometrically ,hence ,gives the measure of the copper concentration of the sample.
Absorbance=Concentration*absorptivity of species(complex)*(path length of light)
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