In a solution, the solute is the minor component that is dissolved in the solvent. For example, salt is the solute in a salt water solution, and isopropanol or ethanol is the solute in a rubbing alcohol solution. Before working out moles of solute, you need to understand what a mole is.
TL;DR (Too Long; Didn't Read)
The number of moles of solute = mass of solute ÷ molar mass of solute, where mass is measured in grams and molar mass (defined as the mass of one mole of a substance in grams) is measured in g/mol.
Concept of Moles
A mole (abbreviated to mol) is a very large number used to measure units (atoms, electrons, ions or molecules), which is equal to 6.022 x 10^23 (the same number of particles as there are atoms in 12 grams of carbon-12). This is known as Avogadro’s number or the Avogadro constant.
Mass of Solute and Molar Mass
The number of moles of solute = mass of solute ÷ molar mass of solute, where mass is measured in grams and molar mass (defined as the mass of one mole of a substance in grams) is measured in g/mol. If you don't know the mass of the solute, weigh it on a scale and record the value.
To find the molar mass of the solute, refer to a periodic table. If the solute is a single element, calculate the molar mass of that element. If it consists of a more than one element (i.e. a compound) calculate the molar mass of the compound.
Every element has a different molar mass. For example, one mole of sodium (Na) has a mass of 22.9898 g/mol. One mole of chlorine (Cl) is 35.4530 g/mol. If your solute is table salt (NaCl), it is a compound of sodium and chlorine. To find the molar mass of NaCl, you add the mass of each element. Work out 22.9898 + 35.4530 = 58.4538.
Moles of Solute
Say you create a solution with 200 grams of table salt. One mole equals the molar mass of the solute which is 58.4538 grams. Divide the mass of the solute by the molar mass to get the number of moles of solute. In this case, work out 200 ÷ 58 = 3.4483 moles of solute.
Calculating Molarity
When you know moles of solute, you can work out molarity (M), the concentration of a solution expressed as the number of moles of solute per liter of solution. To work out molarity, you need to know the total volume of solution as well as the number of moles of solute. Divide the number of moles of solute by the number of liters of solution. For example, if you have 3.4483 moles of table salt in 10 liters of water, work out 3.4483 ÷ 100 = 0.0345. The molarity is 0.0345 M.
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Claire is a writer and editor with 18 years' experience. She writes about science and health for a range of digital publications, including Reader's Digest, HealthCentral, Vice and Zocdoc.