Gen Chem
posted by Sarah H on .
What is n, the number of moles of electrons transferred, in the following reaction?
2 MnO4(aq) + 16 H+(aq) + 10 Cl(aq) ===> 2 Mn2+(aq) + 5 Cl2(g) + 8 H2O(l).

Mn goes from +7 on the left to +1 on the right(per Mn atom).
Cl goes from 1 on the left to zero on the right (for each Cl atom/ion). That is all but telling you the number for n. 
So is n=5 then? (71)+(10)?
or (71)+(10) then n=7?
My choices are 2, 4, 5, 10, 16... 
No. Five is not right.
Notice EACH Mn goes from +7 to +2. That is a change of n = 5 FOR each Mn atom. How many Mn atoms undergo that change? Or look at Cl.
EACH Cl^ goes from 1 to 0. You have ten of them undergoing that change (from 10Cl^ to 5Cl2). What is total n? 
so n=10 then, right? 2Mn atoms (2*5), and 1*10 for the Cl...?

right.
In a redox reaction, electrons lost must equation electrons gained. Each Mn gains 5 electrons (from +7 to +2) and there are two of them and 2 x 5 = 10 electrons gained.
For Cl, we have 10 Cl^ (at 1 each) going to 5Cl2 (at zero each), which is 10 electrons lost.