By Riti Gupta, Updated Mar 24, 2022

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Dealing with extremely large or small quantities can be cumbersome. Scientists address this by standardizing the way they express amounts of matter, simplifying calculations and communication.

The mole (abbrev. mol) is the SI unit for the amount of a substance. It represents roughly 6.022 × 10²³ elementary entities, whether atoms, molecules, ions or particles. Saying “1 mol” is far easier than writing out the full number.

In everyday terms, if someone says there is 1 mol of umbrellas outside, they mean 6.022 × 10²³ umbrellas – the size or shape of the object is irrelevant; a mole is a mole.

What Is an mmol?

An mmol is a millimole – one thousandth of a mole. Thus:

\(1\enspace mmol = 0.001\enspace mol\)

\(1000\enspace mmol = 1\enspace mol\)

Calculating mmol From Grams of Substance

Suppose you have 0.33 g of sodium chloride (NaCl). How many moles, and how many millimoles, does that represent?

First determine the molar mass. From the periodic table, sodium is 22.99 g mol⁻¹ and chlorine is 35.45 g mol⁻¹. Summing gives 58.44 g mol⁻¹ for NaCl.

Use dimensional analysis to convert grams to moles:

\(0.33\,g\,\text{NaCl}\times\left(\dfrac{1\,\text{mol}\,\text{NaCl}}{58.44\,g\,\text{NaCl}}\right) = 5.6\times10^{-3}\,\text{mol}\,\text{NaCl}\)

Because the grams cancel, the result is in moles. To express this as millimoles, multiply by 1000:

\(0.33\,g\,\text{NaCl}\times\left(\dfrac{1\,\text{mol}\,\text{NaCl}}{58.44\,g\,\text{NaCl}}\right)\times\left(\dfrac{1000\,\text{mmol}}{1\,\text{mol}}\right) = 5.6\,\text{mmol}\,\text{NaCl}\)

Thus, 0.33 g of NaCl contains 5.6 mmol – a more convenient figure for many laboratory calculations.

Calculating mmol of a Substance in a Volume of Solution

How many millimoles of nickel(II) sulfate (NiSO₄) are present in 10 mL of a 0.1 M solution?

1. Recognize that “M” denotes molarity, i.e., mol L⁻¹.
2. Convert liters to milliliters: 1 L = 1000 mL.
3. Set up the conversion so only mmol remains:

\(\Bigl(\dfrac{0.1\,\text{mol}\,\text{NiSO}_4}{1\,\text{L}}\Bigr)\times\Bigl(\dfrac{1000\,\text{mmol}}{1\,\text{mol}}\Bigr)\times\Bigl(\dfrac{1\,\text{L}}{1000\,\text{mL}}\Bigr)\times10\,\text{mL} = 10\,\text{mmol}\,\text{NiSO}_4\)

So, 10 mL of a 0.1 M NiSO₄ solution contains exactly 10 mmol of NiSO₄.

When converting to millimoles, always arrange the calculation so that all units cancel except the desired one. This guarantees accuracy and confidence in your results.