How to find empirical formula without percentages

The content that follows is the substance of lecture 9. In this lecture we cover the relationship between Empirical and Molecular formulas and the calculations used to determine one from the other.

The Empirical Formula

An Empirical formula is the chemical formula of a compound that gives the proportions (ratios) of the elements present in the compound but not the actual numbers or arrangement of atoms.

This would be the lowest whole number ratio of the elements in the compound.

For Example:

In order to determine the Empirical formula for a compound or molecule, we need to know the mass percentages of the the elements in the compound. Once we have this information we can convert it to moles to determine the ratios between the elements.

A simple rhyme can be used to remember the process:

Percent to Mass

Mass to Mole

Divide by Small

Multiply 'til Whole

For Example:

NutraSweet is 57.14% C, 6.16% H, 9.52% N, and 27.18% O.  Calculate the empirical formula of NutraSweet and find the molecular formula.  (The molar mass of NutraSweet is 294.30 g/mol)

Start with the number of grams of each element, given in the problem.

If percentages are given, assume that the total mass is 100 grams so that
the mass of each element = the percent given.

Convert the mass of each element to moles using the molar mass from the periodic table.

Divide each mole value by the smallest number of moles calculated.  Round to the nearest whole number.

This is the mole ratio of the elements and is represented by subscripts in the empirical formula.

If the number is too far to round (x.1 ~ x.9), then multiply each solution by the same
factor to get the lowest whole number multiple.

Now, we can find the molecular formula by finding the mass of the empirical formula and setting up a ratio:

The beauty of this little trick is that you conveniently gift yourself with the same number of grams of each elemental component as its contribution to the percent composition. For example, if you assume that you have 100 g of a compound composed of 60.3% magnesium and 39.7% oxygen, you know that you have 60.3 g of magnesium and 39.7 g of oxygen. (The only time you don't do this is if the problem specifically gives you the masses of each element present in the unknown compound.)

Convert the masses from Step 1 into moles using the molar mass.

Determine which element has the smallest mole value. Then divide all the mole values you calculated in Step 2 by this smallest value.

This division yields the mole ratios of the elements of the compound.

If any of your mole ratios aren't whole numbers, multiply all numbers by the smallest possible factor that produces whole-number mole ratios for all the elements.

For example, if you have 1 nitrogen atom for every 0.5 oxygen atoms in a compound, the empirical formula is not N1O0.5. Such a formula casually suggests that an oxygen atom has been split, something that would create a small-scale nuclear explosion. (Kaboom!) Though impressive sounding, this scenario is almost certainly false. Far more likely is that the atoms of nitrogen and oxygen are combining in a 1 : 0.5 ratio but do so in a larger but equivalent ratio of 2 : 1. The empirical formula is thus N2O.

Because the original percent composition data is typically experimental, expect to see a bit of error in the numbers. For example, 2.03 is probably within experimental error of 2, 2.99 is probably 3, and so on.

Write the empirical formula by attaching these whole-number mole ratios as subscripts to the chemical symbol of each element.

Order the elements according to the general rules for naming ionic and molecular compounds.

Here's an example: What is the empirical formula of a substance that is 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass?

For the sake of simplicity, assume that you have a total of 100 g of this mystery compound. Therefore, you have 40.0 g of carbon, 6.7 g of hydrogen, and 53.3 g of oxygen. Convert each of these masses to moles by using the gram atomic masses of C, H, and O:

Notice that the carbon and oxygen mole numbers are the same, so you know the ratio of these two elements is 1:1 within the compound. Next, divide all the mole numbers by the smallest among them, which is 3.33. This division yields

The compound has the empirical formula CH2O. The actual number of atoms within each particle of the compound is some multiple of the numbers expressed in this formula.

How do you find the empirical formula when not given percentages?

Find the empirical formula..

Get the mass of each element by assuming a certain overall mass for the sample (100 g is a good mass to assume when working with percentages). ... .

Convert the mass of each element to moles. ... .

Find the ratio of the moles of each element. ... .

Use the mole ratio to write the empirical fomula..

How do you find empirical formula from percent without mass?

1 Answer. take the percentages divide them by the atomic relative mass of the atoms. After dividing you will get the values. Divide all the values with the smallest value which you get and by doing this you will get a ratio and this will be the empirical formula.

How do you find the empirical formula of a whole number?

Empirical Formula: In Steps In order to find a whole-number ratio, divide the moles of each element by whichever of the moles from step 2 is the smallest. If all the moles at this point are whole numbers (or very close), the empirical formula can be written with the moles as the subscript of each element.