How to Calculate Milliequivalents in 3 Easy Steps

Why is calculation of milliequivalents so difficult? Dare I say, it is often taught in a way that does not foster understanding.

If you are just memorizing equations and canceling out units to calculate milliequivalents we are going to change that. That strategy may be sufficient to pass an exam but isn’t very useful beyond that.

The problem with understanding milliequivalents is that most explanations start with an assumption that key foundational concepts are already established. If they are not then you will have alot of difficulty understanding what numbers are being used in calculating milliequivalents. Calculating milliequivalents is actually quite simple if you understand what you are doing and why.

We are going to make no assumptions of baseline knowledge so that you are left with the whole picture of milliequivalents.

Let’s take care of a few definitions. Terminology is always important. It ensures that we are all on the same page as we move through these concepts.

The Periodic Table

Understanding milliequivalents requires understanding of the organization of the periodic table. The periodic table is organized into groups (columns) and periods (rows).

Ptable.com is the best interactive periodic table. In addition to the Mass and atomic number, it includes the number of atoms per shell for each atom.

Each shell can hold a maximum number of electrons: 2, 8, 8 on the 1st, 2nd and 3rd shell respectively. An atom is considered stable (having low reactivity) when the shells have the maximum number of electrons.

I cannot emphasize enough how important the concept of valency is to understanding milliequivalents and a bunch of other concepts in pharmacology. It is why I created the unit How to Reading Valence. If any of what you just read is confusing, I highly encourage you to read that unit.

What is a Mole?

A mole is a unit of measurement. We use mole when we need to represent a very large number of particles.

1 mole represents the quantity of a substance that contains 6.002 x10²³ particles. This is known as Avogadro’s number. Think about this number (600,200,000,000,000,000,000,000). That’s lot of particles! That’s too big a number to be used in calculations so we use a mole as a surrogate marker for an Avogadro’s number of particles.

Properties of a Mole

Using carbon and oxygen as examples, let us look at the properties of 1 mole of an element.

For individual elements the weight of one mole is simply the mass number/atomic mass. When atoms react to form a stable molecule, like carbon dioxide (CO₂) the weight of 1 mole of the molecule is the sum of the mass numbers of the atoms in the molecule. The weight of one mole of carbon dioxide (CO₂) is 12 + 16 +16 = 44g

Each atom varies in mass and valency (reactivity). Therefore atoms contribute differently in reactions to form stable molecules. We need a way to compare them with and against each other in a way that accounts for those differences. We do this by calculating the equivalent weight.

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Calculating Equivalent Weight

Equivalent weight is a way to standardize moles. While the value of mole and molar mass is sufficient when describing individual elements we need a away to account for the differences in mass and valence when they interact with each other. These differences will affect their contribution or behavior in reactions.

You can think of it as averaging the weight of the particle to its reactivity so that we can make a proportional comparison across the elements.

The ratio of an element’s mass number to valence provides the equivalent weight.

We know that our mass number is the larger number in the periodic table. We know how to calculate valency from the periodic table. If you are still unsure of how to calculate valency please review the unit How to Read Valence.

Let us calculate the equivalent weight of carbon and oxygen.

These calculations are the equivalent weight of 1 mole of oxygen and 1 mole of carbon respectively.

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Calculating Equivalents

Once you know the weight of one equivalent (equivalent weight) we can calculate the number of equivalents in any given quantity of a substance.

The number of equivalents is the ratio of your given quantity in grams to the equivalent weight.

Equivalents to Milliequivalents

Equivalent is usually expressed as milliequivalent simply to provide a number than is easier to read. Milliequivalent is: number of equivalents x 1000

Considering all of we have discussed there are 3 broad steps to calculating milliequivalents:

1. determine the equivalent weight using mass number and valency
2. use equivalent weight to calculate number of equivalents
3. convert equivalents to milliequivalents

Let’s look at a few example calculations for milliequivalents.

Milliequivalent Calculations

If we understand what we are doing, we can tackle any variation of calculating milliequivalent. It allows us to advance the concept of milliequivalents to other calculations like osmolarity and other fluid calculations.

If you have found the unit helpful, I would love to hear from you. Leave your questions or comments below!

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