In chemistry it is important to know the concentration of solutions, and these can be expressed in different ways. The mole is a standard scientific unit that stands for 6.022 x 10 to the power of 23, which is the number of atoms determined experimentally to be found in 12 grams of carbon-12. This number is also called Avogadro’s number. A millimole is one thousandth of this. Parts per million measures the parts of solute per one million parts of solvent, and is often used in environmental measurements. You can convert millimoles to ppm.

- Skill level:
- Moderate

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### Things you need

- Periodic table of the elements
- Calculator

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## Instructions

- 1
Determine the formula of your chemical solution that you know the concentration of in millimoles. For example calcium chloride has the chemical formula CaCl2 which means that there is one atom of calcium (Ca) and two atoms of chlorine (Cl) in each molecule of calcium chloride.

- 2
Use the periodic table to find the atomic weight of each element in the formula. For example the atomic weight of calcium (Ca) is 40.078 and chlorine (Cl) has an atomic weight of 35.453.

- 3
Calculate the molecular weight of the molecule from its chemical formula and the atomic weights. If there is more than one atom of an element you multiply the atomic weight by the number of atoms of that element. For example the molecular weight of calcium chloride is 40.078 + (35.453 x 2) = 110.984 gram.

- 4
Multiply the molecular weight you have calculated by the concentration of the solution in millimoles per litre. This converts the concentration of the solution from millimoles to parts per million, which is the same as the milligrams of chemical per litre of solution. For example 1 millimole of calcium carbonate is the same as 110.98 ppm.

#### Tips and warnings

- There are online calculators for conversions between different measures of concentration (see Resources).