# Error in AAMC 9 Physical Section?

#### imperfection

##### Full Member
10+ Year Member
I've looked over this particular question and the explanation for the answer a hundred times and still keep coming up with them having made a mistake. If you want to see the actual question, it's Q30 in AAMC MCAT Practice Test 9. I've changed the exact reactants and made up new values for E, but the essence of the Q still stands (I really don't want to have legal problems with AAMC...).

The following reaction occurs in a galvanic cell:

2Li(s) + Br2(g) <---> 2Li+(aq) + 2Br-(aq)

With the following E for the half reactions:

Na+(aq) + e- ---> Na(s) -2.43 V
Cl2(g) + 2e- ---> 2Cl-(aq) +1.78 V

Now, the question asks that you calculate the standard emf for the cell.

By my math, this means:
((-1)*(2)*(-2.43)) + 1.78 = 6.64

The (-1) is because the half reaction shown is for the opposite direction. The 2 is because the half reaction shown is for only 1Li+ ion, not two, so we need to multiply the half reaction by two in order to show what's really happening in our cell.

They give the answer as 4.21:
((-1)*(-2.43)) + 1.78 = 4.21

In other words, they don't multiply the E of the Lithium half reaction by two to make up for the difference in quantity between the half reaction and what's happening in the cell. Am I being really stupid here, or is that a mistake on their part?

Thanks so much for any fresh perspectives on this folks! I'm afraid I've been staring at it for way too long...

#### LONeil

##### Full Member
When balancing redox reactions, voltages are NEVER multiplied when equating the electrons between the oxidation and reduction half reactions.

#### LONeil

##### Full Member
I guess I could clarify....reduction potentials are intrinsic properties, thus they aren't changed and are inherent to the cell regardless of the number of electrons being transferred.

#### typicalindian

##### Full Member
5+ Year Member
7+ Year Member
You are NOT EVER supposed to multiply the standard potentials when calculating the standard cell voltage.

About the Ads

#### imperfection

##### Full Member
10+ Year Member
Thanks folks!

This thread is more than 9 years old.

Your message may be considered spam for the following reasons:

1. Your new thread title is very short, and likely is unhelpful.
2. Your reply is very short and likely does not add anything to the thread.
3. Your reply is very long and likely does not add anything to the thread.
4. It is very likely that it does not need any further discussion and thus bumping it serves no purpose.
5. Your message is mostly quotes or spoilers.
6. Your reply has occurred very quickly after a previous reply and likely does not add anything to the thread.
7. This thread is locked.