I would really appreciate some help on how to work out these Chemistry problems
ID: 908491 • Letter: I
Question
I would really appreciate some help on how to work out these Chemistry problems involving electrolysis. THANK YOU VERY MUCH!!!
1. Chromium metal can be electroplated from an acidic solution of (CrO4)^-2
a. Write the half reaction
b. Determine the equivalent weight of Cr
c. What mass of Cr, in grams, will be deposited by a current of 98.54 C
d. What mass of Cr, in grams, will be deposited by a current of 3.0 amps passing for 4 minutes?
e. How many minutes will it take for 1.5 grams of Cr to be deposited by a current of 1.2 amps.
f. How many Faradays of electricity are required to deposit 52 grams of Cr.?
Explanation / Answer
a) Half reaction is:
CrO4 2- + 8H+ + 3e- ------> Cr3+ + 4H2O
b)
Equivalent Weight of Cr3+ = Molecular Wt / 3 (since 3e- are involved)
=> E.W = 52 / 3 = 17.33
c)
Current = 98.54 C
charge of e- = 1.6 x 10^-19 C
Current = charge of e- x no of e-
=> 98.54 = 1.6 x 10^-19 x n
=> n = 6.15875 x 10^20 (number of electrons)
=> Moles of e- = 6.15875 x 10^20 / 6.022 x 10^23 = 1.023 x 10^-3 moles of e-
From the stoichiometry of the half reaction we see that for 3 moles of e- 1 mole of Cr is deposited
=> Moles of Cr deposited = 1.023 x 10^-3 / 3 = 3.41 x 10^-4 moles of Cr
=> Mass of Cr = 3.41 x 10^-4 x 52 = 0.0177 g
d)
Charge = current x time
=> charge = 3 x 4 x 60 = 720 C
=> no of e- = 720 / 1.6 x 10^-19 = 4.5 x 10^21
=> moles of e- = 4.5 x 10^21 / 6.022 x 10^23 = 7.47 x 10^-3
=> Moles of Cr = 7.47 x 10^-3 / 3 = 2.49 x 10^-3 moles
=> Mass of Cr = 2.49 x 10^-3 x 52 = 0.1295 g
e)
Moles of Cr = 1.5 / 52 = 0.0288 moles
=> Moles of e- = 0.0288 x 3 = 0.0865 moles
=> no of e- = 0.0865 x 6.022 x 10^23 = 5.211 x 10^22
=> charge = 5.211 x 10^22 x 1.6 x 10^-19 = 8338.15 C
=> Time = 8338.15 / 1.2 = 6948.5 sec = 115.8 minutes
f)
Moles of Cr = 52 / 52 = 1
=> Moles of e- = 3
=> no of e- = 3 x 6.022 x 10^23 = 1.8066 x 10^24
=> charge = 1.8066 x 10^24 x 1.6 x 10^-19 = 289056 C
=> Faraday's of electricity = 289056 / 96485 = 3