Normality! What is going on here?

[sizillyd]

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Okay, so here is the problem

Consider the reaction of 2.5M dichromate:

Cr2O7^2- ---> Cr^3+ (acidic medium)

What would be the solution Normality if the reaction goes to completion?

The answer is 7.5M

Here is the sad thing, it explains how to do it, but confuses me.

The explanation is:

N is greater than or equal to M, so we would be able to automatically eliminate values of N lower than 2.5. Then it says to balance the equation, and keep in mind that it is in an acidic medium so use H+ to balance the water.

It says that Cr goes from a 6+ oxidation state to 3+ oxidation state, so that is a difference in 3 e-. Then they say to multiply 3 by the Molarity and you get the answer.... 7.5M

Why did they multiply by 3? I see that there is a difference in 3 e-, but WTF? I know the equation for normality is something like:

# of gram equivalent solute
---------------------------
Liters solvent

Can anyone help me with this? Sorry for butchering any logic here...thanks

 

[venacontracta]

Ignore the equation you have for normality. It is correct but confusing and unnecessary.

Normality is "molarity of the important species"

In a redox reaction, the important species is electrons. For every mole of dichromate, you need three moles of electrons. So in this situation, the normality of the dichromate solution is three times the molarity.

For an acid, the important species is the proton. So for a monoprotic acid, eg HCl, normality = molarity. For a polyprotic acid, normality = molarity x the number of acidic protons on each acid molecule (e.g. a 3M solution of H2SO4 is 6N)

For a base, the important species is the hydroxide ion. So 1M NaOH = 1N and 1M Ca(OH)2 = 2N

You won't see normality in any other situations than these on the MCAT.

Okay, so here is the problem

Consider the reaction of 2.5M dichromate:

Cr2O7^2- ---> Cr^3+ (acidic medium)

What would be the solution Normality if the reaction goes to completion?

The answer is 7.5M

Here is the sad thing, it explains how to do it, but confuses me.

The explanation is:

N is greater than or equal to M, so we would be able to automatically eliminate values of N lower than 2.5. Then it says to balance the equation, and keep in mind that it is in an acidic medium so use H+ to balance the water.

It says that Cr goes from a 6+ oxidation state to 3+ oxidation state, so that is a difference in 3 e-. Then they say to multiply 3 by the Molarity and you get the answer.... 7.5M

Why did they multiply by 3? I see that there is a difference in 3 e-, but WTF? I know the equation for normality is something like:

# of gram equivalent solute
---------------------------
Liters solvent

Can anyone help me with this? Sorry for butchering any logic here...thanks

 

[DrDre2001]

Ignore the equation you have for normality. It is correct but confusing and unnecessary.

Normality is "molarity of the important species"

In a redox reaction, the important species is electrons. For every mole of dichromate, you need three moles of electrons. So in this situation, the normality of the dichromate solution is three times the molarity.

For an acid, the important species is the proton. So for a monoprotic acid, eg HCl, normality = molarity. For a polyprotic acid, normality = molarity x the number of acidic protons on each acid molecule (e.g. a 3M solution of H2SO4 is 6N)

For a base, the important species is the hydroxide ion. So 1M NaOH = 1N and 1M Ca(OH)2 = 2N

You won't see normality in any other situations than these on the MCAT.

I agree with this.

Normality is pretty simple ("molarity of the important species" is a great and simple way to describe it), but the way science textbooks give you an equation and try to explain it often confuses people.


It is like formal charge in a sense. The equation is: unpaired electons + paired electrons/2 = formal charge. But, it's a lot easier and quicker if you just stop using the equation and realize that formal charge just tries to give ownership of the electrons to each atom.

I've seen other students get wrapped up in the equations for normality and formal charge, instead of just understanding what the purpose of the two are.