Chemists regularly use both moles and liters as units to describe the quantity of chemical substances. However, there is a fundamental difference between the two. Moles describe a standard quantity of atoms or molecules of a substance.
The number of particles in a mole is sometimes referred to as Avogadro's number and is very large, typically represented as: 6.02 × 1023. This number was determined in the 1800's by mathematicians and physicists through studying kinetic molecular theory and Brownian motion. It is important to be able to determine the number of particles in some unit of gas in order to calculate the mass of the gas.
Liters, however, are a measure of volume used in the metric system. But while there is no such thing as a liters to molecules calculator, you can convert from liters to moles or mL to moles if you know the density of your chemical and if you first calculate its molecular weight.
How to Convert Liters to Moles
Make sure that the value of density you use in your calculations is for appropriate temperature, since the density of a compound can change substantially with temperature.
Write out the chemical formula of the chemical you are converting from liters to moles. This formula shows how many types of atoms are in each molecule as well as how many of each type. The chemical cyclohexane, for example, has the formula C6H12.
Look up the atomic weight of each element in the chemical formula, using a periodic table. For cyclohexane, you would look up the atomic weight of carbon (C), which is 12.01 and hydrogen (H), which is 1.008.
Multiply the atomic weight of each element by the number of atoms of that element in the formula, then add all the resulting products. This value is the gram molecular weight of the chemical, in units of grams per mole. In the case of cyclohexane, you would calculate (12.01)×(6) + (1.008)×(12) = 84.16 grams per mole.
Multiply the volume of the compound, in liters, by 1,000. This will convert the volume into units of milliliters. If you had 2 liters of cyclohexane, for example, you would convert this as follows; 2 x 1000 = 2,000 milliliters.
Multiply the volume of your chemical, in milliliters, by the density value, in grams per milliliter. This calculation gives you the weight of the compound, in grams. Labs and technical reference sources commonly use units of grams per milliliter to describe the density of substances.
You can find the density of your compound in the "physical properties" section of the Material Safety Data Sheet which the manufacturer provides with the chemical. The density of cyclohexane is 0.78 grams per milliliter, so the weight of cyclohexane in 2,000 milliliters is (2000)(0.78) = 1,560 grams.
Divide the weight you just calculated by the gram molecular weight of your chemical, in grams per mole. The result is the number of moles of the compound in the number of liters you started with at the beginning of your calculations. In the case of the example, the moles of cyclohexane are found by the calculation 1,560 grams/84.16 grams/mole = 18.5 moles.
It is common to use molarity, or the moles of a solute that is dissolved into a solvent to describe the concentration of a chemical compound. By using these units, it is easy to determine how concentrations needs to be adjusted by adding more solvent, or solute.
Despite the name, Avogadro's number was not determined by Amadeo Avogadro, but referred to as such by those seeking to find the correct estimate of particles in a mole of gas. The first to use the term was a French physicist, Jean Baptiste Perrin.
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