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What is Normality? | ChemistryTwig

What is Normality?

normalityNormality is used in chemistry to denote the strength of a solution. It’s similar to, and sometimes even the same as, molarity. Normality has to be one of the most misunderstood units of measure. It’s true that normality can be pretty ambiguous. I’ll go over a brief overview of normality to get you jump-started on this uncommon unit of measure.

What’s the definition of normality? In chemistry, normality is defined as molar concentration divided by an equivalence factor. What the heck does that mean?!

Turns out, normality isn’t really that confusing. It just has a super confusing definition. Normality is also known as equivalent concentration. This is because of that “equivalence factor” mentioned in its definition.

First, you need to choose a species to define the “equivalence” that you want to determine. For example, let’s choose H+ as our species. Consider the following solution:

1 Molar (1 M) HCl = 1 mole/L of HCl

Now, we can define the solution in terms of H+. That means we must convert to an equivalent concentration, or normality.

Since 1 mole of HCl dissociates into 1 H+ and 1 CL, that means there is 1 equivalent H+ per 1 mole of HCl. That means:

1 Molar (1 M) HCl = 1 N H+ = 1 Normal H+ = 1 N HCl in terms of equivalence H+

Now, choose a different acid. This is where things can start to get tricky. Let’s take the following example:

1 Molar (1 M) H2SO4 = 1 mole/L of H2SO4

Since 1 mole of H2SO4 dissociates into 2 H+, that means there are 2 equivalent moles of H+ per 1 mole of H2SO4. That is:

1 Molar (1 M) H2SO4 = 2 Normal H+ = 2 N H2SO4 in terms of equivalence H+.

You may still wonder how normality can be ambiguous. Consider the 1 mole/L solution of H2SO4. Now, instead of equivalent H+ how about we consider it in terms of equivalent SO4. Since 1 mole of H2SO4 dissociates into 1 SO4, that means there is 1 equivalent mole of SO4 per 1 mole of H2SO4:

1 Molar (1 M) H2SO4 = 1 Normal SO4 = 1 N H2SO4 in terms of equivalence SO4.

{adinserter 3}See how it can get confusing? Nevertheless, normality has its merits. Since normality is always defined as an equivalent concentration, it never matters what the solute is as long as you know the concentration of the species of interest. For example, imagine you need to know the hydrogen ion concentration of a solution that contains HCl, H2SO4, and H3PO4 since you are able to correlate pH to H+ concentration. Instead of measuring the concentration, you can simply measure the H+ ion concentration of the solution. Then, say someone adds 1000 liters of a 2 N H+ solution to the tank. It doesn’t matter what the exact concentration of HCl, H2SO4, and H3PO4 in the solution because you know it is 2 N of equivalent H+. That means:

1000 L x 2 N (or 2 mole/L) H+ = 2000 moles of H+

Super easy to add!

I hope this has cleared up any questions to do with normality. Normality is a tough chemistry concept to grasp. However, just like with all other chemistry concepts, it’s not difficult once you take the time to figure it out.

Please leave us a comment if you need any more clarification or just want to share your struggles with normality!

[Image Credit] Horia Varlan license: Image License Image was modified by ChemistryTwig, Copyright 2014

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