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The simplest formula of a compound is the smallest set of possible whole numbers for the atoms in a molecule. It is not necessarily the same as the molecular formula: for example, the simplest formula for benzene is CH, but benzene's actual molecular formula is C6H6. The molecular formula is always an integer multiple of the simplest formula. Some examples are shown in the table below.
 Compound Simplest formula Molecular formula Multiple Water H2O H2O 1 Hydrogen Peroxide HO H2O2 2 Glucose CH2O C6H12O6 6

Many analytical instruments designed to give the compostion of a molecule give either the total mass of each element or the percentage mass for each. The procedure to convert this data back into a simplest formula is fairly straightforward.

1. Convert the mass of each element into the number of moles of that element. (If given mass percents, convert these to grams by multiplying by 100 grams of material: this will give you a set of masses of each element.)
2. Divide all the moles by the smallest number of moles: this will give you the mole ratio
3. If the mole ratio is in whole numbers, you are done. If there are still fractions, multiply all the ratios through by the smallest number that removes the fraction
To get the molecular formula, you must find some other method that measures the mass of the molecule directly.

Example: A substance has the following elemental composition: 17.55% Na, 39.70% Cr and 42.75% O. What is the simplest formula for this compound?

Solution: For this problem, we've been given mass percents: we can convert these back into masses by picking an amount of the substance, say 100 grams, and multiplying through to get the masses of each element in 100 grams of the compound.

17.55% Na * 100 g = 17.55 g Na
39.70% Cr * 100g = 39.70 g Cr
42.75% O * 100 g = 42.75 g O
Next, we convert each of these to a number of moles by dividing by the molecular weight
17.55 g Na/22.99 g/mole = 0.7634 moles Na
39.70 g Cr/52.00 g/mole = 0.7634 moles Cr
42.75 g O/16.00 g/mole = 2.672 mole O
Now, divide each by the smallest number of moles to get the mole ratio: the smallest number is 0.7634 moles
0.7634 moles Na/0.7634 moles = 1 Na
0.7634 moles Cr/0.7634 moles = 1 Cr
2.672 moles O/0.7634 moles = 3.5 O
If these were whole numbers we'd be done: the formula would be the ratio above. Since they are not, we need to multiply by an integer. If we multiply by 2, the 3.5 turns into 7, and we'll have a nice integer ratio:
1 Na * 2 = 2 Na
1 Cr * 2 = 2 Cr
3.5 O * 2 = 7 O
The simplest formula is Na2Cr2O7, sodium dichromate.

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