PRACTICE PROBLEMS 1) What is the oxidation number of manganese in the MnO41 ion?

Question

PRACTICE PROBLEMS 1) What is the oxidation number of manganese in the MnO41 ion? of manganese in the MnO41-ion? B)+2 C)+5 D) +4 E) +7 2) How long (in hours) must a current of 5.0 amperes be maintained to electroplate 60 g of calcium from molten CaCla? A) 27 hours B) 8.3 hours C) 11 hours D) 16 hours E) 5.9 hours 3) what is G, per mole of dichromate ions for the reduction of dichromate ions, CrO, to Cr* by bromide ions, Br, in acidic solution? (Hint: Use the standard cell potential.) (a) +26.3 kJ (b)-145 kJ (c) +145 k) (d) -26.3 k) (e) -53.6 kJ 4) Estimate the equilibrium constant for the system indicated at 25°C. A)-10 B)-102 C)-104 D)-10 E)-10-2 5) Calculate the potential (in volts) for the voltaic (or galvanic) cell indicated at 25 C. Ga/ Ga (10 M) II Ag (104 M)/Ag A) 1.29 v B) 0.97 V C) 1.45 V D) 1.21 v E) 1.37 v 6) What is the coefficient of the iodide ion when the following equation is balanced? Mn04 (aq) + I-(aq) Mn2+ (aq) + 12 (s) (acidic solution)

Explanation / Answer

1.

MnO4 = -1 and O = -2 always

(4)(-2) + Mn = -1

Mn = +7

2.

m = 60

mol = 60/110 = 0.54545

then

1 mol of Ca+2 requires 2 mol fo e- so

0.54545 --> 2*0.54545 = 1.0909 mol of e-

I = 5

C = 96500*1.0909 = 105271.85 C

then

I = C/t

t = C/I = 105271.85/5 = 21054.37 s = 5.848436 hours

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