How To Calculate Normality In Chemistry

When you hear the word "normality," science may not be the first thing that comes to mind. "Normal," to most people, means "ordinary" or "typical," and doesn't seem to have a whole lot of scientific weight. But in chemistry, the term normality is closely related to a few core chemistry concepts, and in particular, it is vital in the area of solutions chemistry.

If you see "normal" as "expected," even in the chemical sense this is more or less on target: A normalized preparation is one that has been created in proportion or relation to an established standard. To discover how to calculate the normality of NaOH, or how to convert from normality to molarity (another core chemistry concept), read on!

First, it is necessary to understand how atoms and molecules combine in chemical reactions, which is not quite like the "ingredients" in, say a cooking recipe or a construction project. Ordinarily, you would keep track of different objects by measuring their masses (in kilograms, pounds or ounces, for example) or perhaps their volumes.

So, if you know that 1 kg of flour and 0.5 kg of sugar is enough to make 10 servings of a particular dessert product, you know that 100 kg of flour and 50 kg of sugar are enough for 1,000 servings. And in mixing drinks, if 50 mL of your own homemade fitness water needs 5 mL of vinegar, then 1,000 mL (1 L) of the product needs 20 times this amount of of the vinegar reagent is 100 mL.

With reactions, it's whole particles (atoms and molecules) you need to count. These are measured in moles, and translating them into this "language" from mass only requires you to know the molar masses of the constituent atoms.

A mole of anything, such as atoms, molecules, marbles or giraffes, is equal to 6.022 × 10²³ individual instances of that thing. This happens to be the number of particles in exactly 12 grams (g) of the most common form of the element carbon, number 6 on the periodic table of elements. The number of grams in 1 mole (mol) of a given element is under its symbol in its personalized "box" on the table.

To get the molar mass of a molecule, or the mass of 1 mol of those molecules, simply add the individual masses of the atoms, being sure to account for subscripts. So for a water molecule, H₂O, you would add the mass of 1 mol O (15.999 g) to the mass of 2 mol H (2 × 1.008 g, or 2.016 g) to get about 18.015 g.

Concentration in moles per liter (mol/L) is called molarity, denoted M.

You were told above that in chemical reactions, atoms combine in terms of moles, which you now can get from their masses. For example, in a calcium chloride (CaCl₂) solution, the dissociation reaction can be expressed:

CaCl₂(aq) ⇌ Ca²⁺(aq) + 2Cl^–(aq)

Thus, 1 mol of the reactant yields 1 mol Ca²⁺ ions but 2 mol of Cl^– ions.

Normality is a measure of concentration equal to the gram equivalent weight (EW) of solute per liter of solution. The normality formula of interest is N = Mn.

Here, n is the number of equivalents, or the number of single-charge ions the species can react with.

So, to convert normality to molarity or vice-versa in the case of CaCl₂, you know that a 1 M solution will generate a 2 M solution of chloride ions and a 1 M solution of Ca²⁺ ions, which because of their charge also have a value of 2 for n. So in this case, N = (1 M)(2) = 2N.

See the Resources for a handy way to handle tasks such as these for a variety of combinations of acids and bases.