Converting moles to grams or grams to moles is something you do all the time in chemistry. Chemical equations relate quantities of reactants and products using moles, yet you measure chemicals on scales and balances, which use grams. The conversion is easy, providing you set it up right.
How to Convert Moles to Grams
All you need is a periodic table for looking up the atomic masses of the elements.
Grams = Moles x Atomic Mass
g = mol x (g/mol)
Here are the steps for converting moles to grams:
- Start with the number of moles and the chemical formula of the substance.
- Find the molar mass of the substance. Add together the atomic mass of each element multiplied by its subscript. If there is no subscript, use the atomic mass of that element.
- Multiply the number of moles by the molar mass. This gives an answer in grams.
Moles to Grams Example #1
For example, find the mass in grams of 3.6 moles of sulfuric acid (H2SO4).
First, look up the atomic masses of the elements:
- H = 1.008
- S = 32.06
- O = 16.00
Now, use the chemical formula and find the molar mass of H2SO4. You multiply the atomic mass of each element by its subscript. Remember, if there is no subscript, it’s the same as multiplying by 1. Add up the masses of each element according to the chemical formula.
molar mass of H2SO4 = (1.008)(2) + 32.06 + (16.00)(4) = 2.016 + 32.06 + 64.00 = 98.076 grams per mole (g/mol)
Finally, multiply the number of moles by the molar mass:
mass of 3.6 moles H2SO4 = 3.6 moles x 98.076 grams/mole = 353.07 grams
Moles to Grams Example #2
Find the mass in grams of 0.700 moles of hydrogen peroxide (H2O2).
This time let’s pay attention to the number of significant figures. You have 3 significant figures in 0.700 moles.
Write down the atomic masses of the elements:
- H = 1.008
- O = 15.999
Calculate the molar mass of H2O2:
molar mass of H2O2 = (1.008)(2) + (15.999)(2) = 2.016 + 31.998 = 34.014 g/mol
Now, multiply the moles of hydrogen peroxide by its molar mass for the answer in grams:
grams of H2O2 = 0.700 moles x 34.014 grams/mole = 23.810 grams
Grams to Moles Conversion
Converting grams to moles is just as easy. Once again, you use the molar mass of the substance.
Moles = Grams / Atomic Mass
mol = g x mol/g
- Start with the number of grams and the chemical formula.
- Find the molar mass of the substance.
- Divide the mass by the molar mass for an answer in moles.
For example, find the number of moles in 25.0 grams of potassium permanganate (KMnO4).
Using a periodic table, look up the masses of the elements:
- K = 39.01
- Mn = 54.94
- O = 16.00
Calculate the molar mass of KMnO4:
molar mass of KMnO4 = 39.01 + 54.94 + (16.00)(4) = 157.95 g/mol
Divide the mass in grams by the molar mass (g/mol) and get moles:
moles of KMnO4 = 25.0 g / 157.95 g/mol = 0.158 moles
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- Barański, Andrzej (2012). “The Atomic Mass Unit, the Avogadro Constant, and the Mole: A Way to Understanding”. Journal of Chemical Education. 89 (1): 97–102. doi:10.1021/ed2001957
- Cooper, G.; Humphry, S. (2010). “The ontological distinction between units and entities”. Synthese. 187 (2): 393–401. doi:10.1007/s11229-010-9832-1
- International Bureau of Weights and Measures (2006). The International System of Units (SI) (8th ed.). ISBN 92-822-2213-6.
- “Weights and Measures Act 1985 (c. 72)”. The UK Statute Law Database. Office of Public Sector Information.