Question

What is the empirical formula for a compound that is 24.7 % calcium, 1.20 % hydrogen, 14.8 % carbon, and 59.3 % oxygen?

Answer 1

For problems like this, where they give you the % composition of an unknown compound, the process is generally the same:

1. assume you have 100g of the compound. therefore, if it's 24.7% calcium, you have 24.7g calcium, etc. for the other elements

2. for each element, divide grams/molar mass to get the # of moles

3. once you have the # of moles for each compound, look at the smallest mole #. divide every other mole amount by that number.

as an example, if you find that you have the least moles of carbon, you'd divide

moles calcium/moles carbon
moles hydrogen/moles carbon, etc.

to get the lowest mole ratio

4. at this point, if you have whole numbers (or close to whole numbers, like 2.99 or 1.01) you can use that ratio of elements to write your empirical formula.

5. if you have fractional components like 0.5 moles or 1.33 moles, multiply all the mole numbers by the lowest whole number that will convert them all to whole numbers. ex. if you have 0.5 moles you'd multiply everything by 2 to convert it to a whole number. from there, you'd write the empirical formula from the mole ratio