| H is 1.01 grams per mole O is 16.00 grams per mole |
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 | OK, lets take water, or H20 as an example. First, find the molar mass of water by adding up the atomic masses of the elements. Thats easy. You can use your handy pocket-sized laminated periodic table. | |
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| Oh, I get it. their mass numbers. | |
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| molar mass = (2 x 1.01) + 16.00 molar mass = 18.02 |
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| Get the molar mass by tallying all the masses of elements in the compound. You have to look at the subscripts and note that after the hydrogen (H), the 2 indicates there are two atoms of hydrogen. There arent any subscripts after oxygen (O), which means only one atom needs to be counted. | |
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| I'm still with ya... | |
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| mass % H = (2 x 1.01) / 18.02 x 100% mass % H = 11.19% mass % O = 16.00 / 18.02 mass % O = 88.81% |
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| Now, divide the mass of each element by the total mass to get the mass percentages: | |
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| Looks like the answer is up there in the sky! | |
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