1-Purification of chromium can be achieved by electrorefining chromium from an i
ID: 975431 • Letter: 1
Question
1-Purification of chromium can be achieved by electrorefining chromium from an impure chromium anode onto a pure chromium cathode in an electrolytic cell. How many hours will it take to plate 17.5 kg of chromium onto the cathode if the current passed through the cell is held constant at 35.5 A? Assume the chromium in the electrolytic solution is present as Cr3 .
Answer must be in HOURS.
2-Calculate the pH at the equivalence point for the titration of 0.240 M methylamine (CH3NH2) with 0.240 M HCl. The Kb of methylamine is 5.0× 10–4.
___pH
Explanation / Answer
2) Suppose 1 L of CH3NH2 .
To reach the equivalence point,
since the concentrations of HCl and CH3NH2 are the same we need 1 L of HCl
Total volume = 2 L
At the equilvalence point 0.24 mol CH3NH2 + 0.24 mole H+ ----> 0.24 mole CH3NH3+
[CH3NH3+] = 0.24 mol/ 2 L = 0.12 M
Givan that Kb = 5.0× 10–4
Ka = Kw/Kb = 1.0 x 10^-14 / 5.0 x 10^-4 = 2.0 x 10^-11
CH3NH3+ <-----> CH3NH2 + H+
Ka = [CH3NH2] [H+] / [CH3NH3+ ]
2.0 x 10^-11 = x^2 / 0.12-x
Since x is very small,
2.0 x 10^-11 = x^2 / 0.12
x = 1.55 x 10^-6 M
Hence,
[H+]= 1.55 x 10^-6 M
Then,
pH = - log [H+] = - log [1.55 x 10^-6 ] = 5.8
Therefore,
pH at the equivalence point = 5.8
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