Normality

Dr. Steve Badger

Concentrations of Solutions

A brief review of molarity & molality Molarity = Number of moles of solute Liter of solution Molality = Number of moles of solute Kg of solvent

Preparing a Solution of Known Molarity

Would you use a volumetric flask to prepare a solution of known molality?

How would you prepare a solution of known molality?

Another way to express solution concentration

• Now that you understand molarity… Molarity = Number of moles of solute Liter of solution …let’s consider normality!

Normality = Number of equivalents of solute Liter of solution

What’s an equivalent ?

• An equivalent of a substance is the mass (grams) of that substance that will combine with one mole of another reactant.

• In an acid-base reaction, an equivalent is that amount of a substance that reacts with or liberates 1.0 mole of H + .

What’s an equivalent ?

• An equivalent of a substance is the mass (grams) of that substance that will combine with one mole of another reactant.

• In a redox reaction, an equivalent is that amount of a substance that gains or loses 1.0 mole of e – s .

Making a 1.00N Solution

• Calculate the mass of one equivalent of the substance, then measure that number of grams of the substance.

• Put that substance in…. ??

• Add how much solvent?

Preparing a Solution of Known Normality

AbNormality?

• Why don’t many modern chemistry textbooks cover normality?

• If we take a dimensional analysis approach to problems solving, normality is a superfluous concept.

• Consider the following problem:

What volume of a 0.500 M KOH solution is required to titrate 10.0 mL of a 0.20 M H 2 SO 4 solution?

H 2 SO 4 + KOH H 2 O + K 2 SO 4 Could we just use this equation?

M

a V a = M b V b

Here’s what we’d get if we used

M

a V a = M b V b

0.20 M H 2 SO 4 X 10.0 mL = 0.500 M KOH X ? mL

Solving this, we get 4.0 mL of 0.500 M KOH.

But is this correct?

No,

it’s wrong!

Why?

So let’s see how we solve this correctly.

What volume of a 0.500 M KOH solution is required to titrate 10.0 mL of a 0.20 M H 2 SO 4 solution?

BALANCE THE CHEMICAL EQUATION!

H 2 SO 4 + KOH H 2 O + K 2 SO 4

M

volume acid acid rx moles acid coef.

moles base

M

base volume base

What volume of a 0.500 M KOH solution is required to titrate 10.0 mL of a 0.20 M H 2 SO 4 solution?

H 2 SO 4 + 2 KOH 2 H 2 O + K 2 SO 4

M

volume acid acid rx moles acid coef.

moles base

M

base volume base 10.0 mL x ___ mol H 2 SO 4 ____ mL soln x __ mol KOH __ mol H 2 SO 4 x ____ ml soln ____ mol KOH = ___ mL

What volume of a 0.500 M KOH solution is required to titrate 10.00 mL of a 0.20 M H 2 SO 4 solution?

H 2 SO 4 + 2 KOH 2 H 2 O + K 2 SO 4

M

volume acid acid rx moles acid coef.

moles base

M

base volume base 10.0 mL x 0.20 mol H 2 SO 4 1000 mL soln x 2 mol KOH 1 mol H 2 SO 4 x 1000 ml soln 0.500 mol KOH = 8.0 mL

Notice this!

volume acid

M

acid moles acid rx coef.

moles base

M

base volume base 10.0 mL x 0.20 mol H 2 SO 4 1000 mL soln x 2 mol KOH 1 mol H 2 SO 4 x 1000 ml soln 0.500 mol KOH = 8.0 mL Notice this!

This is what makes normality superfluous!

Is the following statement true or false?

• Any volume of a base will completely react with (neutralize) that same volume of acid if the two solutions have the same normality (vice versa too) .

• In other words, is this true:

N

a V a = N b V b

Is the following statement true or false?

• Any volume of a reducing agent will completely react with that same volume of an oxidizing agent if the two solutions have the same normality.

• In other words, is this true:

N

ox V ox = N red V red

Let’s solve a few problems using normality

• Look at the handout that has sample problems and work the first one.

And another thing…

• We also use equivalents and milliequivalents as an amount of a substance • The same way that we use moles (mol) and millimoles (mmol) as an amount of a substance

If you need more help:

Come by my office and I can give you a few pages of worked examples from a Schaum’s Solved Problems Series. You can download this PowerPoint file at my EU web site: www.evangel.edu/Personal/badgers/Web/

The End

Now wasn’t that fascinating?