Stoichiometry: Convert Grams to Moles (3 of 5)

Step-by-Step Example: Convert 9.62 grams of H2O to moles. (watch video)

  1. Circle and label what you are given and what you are trying to find.
         Given: 9.62 of H2O. Trying to find: number of grams of H2O.
  2. Find the GFM for the substance (in this case H2O). For water the GFM is 18.02 g/mol.
  3. Set up the math in the following format:
         9.62 g H2O    x    1 mole H2O/18.02 grams
  4. Cancel out units and multiply.
         9.62 grams H2O    x    1 mole H2O/18.02 grams    =    (9.62 x 1 mole) / 18.02    =    0.53 moles
  5. Check to see if your answer makes sense. Here we know that 1 mole of H2O is about 18 grams (look at the GFM). So 9 grams should be a about one half of a mole.

Remember: when converting grams to moles: divide by the GFM.


Grams to Moles: Some Notes

Converting from grams to moles is the reverse process you used to go from moles to grams. Here again we aren't changing quantities, just the units. It's a good idea to estimate before converting from grams to moles.

Try to estimate what the answer should be before you do the calculation. If I'm asked to 10 grams of H2O to moles I can:

  1. calculate the GFM for water (it’s 18.02 g/mol). Using the periodic table I can see that one mole of water is going to have a mass of 18.02 grams.
  2. compare the number of grams given with the GFM. I'm given 10 grams to convert to moles. Since one mole has a mass of 18.02 grams then I must have less than a mole (10 is less than 18.02). I'd guess a bit less than 1/2 mole – maybe 0.40 moles.

Also remember that when we went from moles to grams we multiplied (or mole-tiplied if it helps you remember). Converting from grams to moles we’ll do the opposite: divide.
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Note: We know that 1 mole of water = 18.02 g.

So 1 mol H2O/18.02 g AND
18.02 g/ 1 mol H2O both equal one.

For converting from grams to moles use the factor with grams on the bottom.

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