Lesson 7:
Acids and Bases


 

Normality

Introduction

While pH is used to record the acidity or alkalinity of natural waters, we use a measurement known as normality to show the concentration of the much stronger acid and base solutions we use in the lab.  Normality is based on molarity, but also takes into account a characteristic of acids and bases which we will call "equivalents" and will describe in the next section. 



Equivalents

Although molarity can be used to measure the concentration of acids, it is a relatively unuseful measurement for understanding neutralization reactions.  Why?  Because not every acid or base can add (or remove) the same number of hydrogen ions from solution.  Let's consider two different acids:

Reactions of hydrochloric acid
Reactions of sulfuric acid
HCl yields H+ + Cl- H2SO4 yields H+ + HSO4-

HSO4- yields H+ + SO4-2

As we see in the left side of the table, one molecule of hydrochloric acid adds one hydrogen ion to the solution.  On the other hand, sulfuric acid releases one hydrogen ion, as shown in the first equation, then ionizes again, releasing a second hydrogen ion.  So one molecule of hydrochloric acid could neutralize one molecule of a base while one molecule of sulfuric acid could neutralize two molecules of a base:

HCl + NaOH
yields H2O + NaCl

H2SO4 + 2NaOH yields 2H2O + 2NaSO4

When talking about the concentration of acids or bases, we use measurements which involve equivalents.  An equivalent is the number of moles of hydrogen ions one mole of an acid will donate or one mole of a base will accept.  For example, hydrochloric acid has an equivalent value of 1 because each molecule of acid donates only one hydrogen ion, so one mole of hydrochloric acid will donate one mole of hydrogen ions.  Sulfuric acid, on the other hand, has an equivalent value of 2.  To give you a couple more examples, sodium hydroxide has an equivalent value of 1 while calcium hydroxide has an equivalent value of 2. 

Let's simplify equivalents here:

For instance, you have calcium hydroxide, Ca(OH)2. You can see there are 2 oxygen ions and 2 hydrogen ions, therefore the equivalent is 2. Another instance, sulfuric acid which is listed above, H2SO4. You can see there are 2 Hydrogen ions and 4 Oxygen ions. Since you have to have equal amounts to remove the ions, this equivalent will be 2. If you had 3 Hydrogen ions and 4 Oxygen ions, the equivalent would be 3 because you had enough oxygen ions to take care of the 3 hydrogen ions. Sodium Hydroxide, NaOH, has an equivalent of 1 since there is 1 Oxygen ion and 1 Hydrogen ion. You have to be able to have one oxygen ion for every hydrogen ion you are trying to get rid of.



Normality

Normality is the most common measurement used for showing the concentration of acids and bases.  Normality takes into account both the molarity of the solution and the equivalent content of the acid or base, using the equation shown below:

Normality (N) = Molarity (M) × Equivalent (N/M)

If you cancelled units in the equation above, you would find that normality is equal to the number of moles of acid or base per liter. 

Let's consider a 0.5 M solution of HCl.  Since we know that one mole of HCl contains 1 equivalent acid, we can calculate normality as follows:

Normality = (0.5 M) × (1 N/M)

Normality = 0.5 N

For all acids and bases with an equivalent value of 1, the normality of the solution will be equal to the molarity of the solution. 

But not every acid and base will have a normality equal to its molarity.  How about a 3 M solution of barium hydroxide?  Barium hydroxide releases two hydroxide ions per molecule of the base, so the equivalent value is 2.  As a result, we would calculate normality as follows:

Normality = (3 M) × (2 N/M)

Normality = 6 N




Calculating Normality From Grams

To calculate the normality of a solution you are preparing, you need to combine the equation for calculating molarity and the equation for calculating normality.  To simplify matters, we've combined the two equations for you:


Formula for calculating normality

So what would the normality of the solution be if we dissolved 6.80 grams of calcium hydroxide in water to produce a 0.50 L solution?  First, we have to calculate the molar mass of calcium hydroxide -  74.10 g/mol.  Then we have to figure out the equivalent value of calcium hydroxide - 2.  And, finally, we can just plug numbers into the equation:

calculation

The normality of the resulting solution would be 0.37 N.



Calculating Dilutions

Once you have calculated the normality of an acid or base solution, you can easily calculate the concentration of any dilutions of that solution.  The formula used is essentially the same as that used for any other dilution calculation:

N1V1 = N2V2

Where:
N1 = normality of the first solution
V1 = volume of the first solution
N2 = normality of the second solution
V2 = volume of the second solution

For example, after producing the 0.5 L of a 0.37 N solution of calcium hydroxide in the last section, how would you dilute it to form a 0.25 N solution? 

(0.37 N) (0.50 L) = (0.25 N) V2

0.74 L = V2

Based on the calculations above, we know that we have to add enough water to the 0.37 N solution so that the total volume reaches 0.74 L.  Then we would have a 0.25 N solution. 



Converting from Other Units

Converting the concentration of a solution between normality and molarity was explained in an earlier section.  We can combine the equation introduced there with the equations introduced in Lesson 4 to convert from normality to the other types of concentration (assuming aqueous solutions.)


convert to mg/L


converting to ppm


convert to %


For example, if you had a solution of 1 N strontium hydroxide, what is its percent concentration?

calculations



Part 4: Alkalinity