Determining an empirical formula from percent composition involves a step-by-step method to convert percentages into the simplest whole-number ratio of atoms. Here’s how you do it:
Steps in words (no equations):
- Write down the percent composition of each element in the compound.
- For example, suppose a compound has 40% carbon, 6.7% hydrogen, and 53.3% oxygen.
- Assume you have 100 grams of the compound.
- This way, the percentages become grams directly (40 g carbon, 6.7 g hydrogen, 53.3 g oxygen).
- Convert grams to moles for each element by dividing the given mass by the atomic mass of that element.
- This gives the number of moles of carbon, hydrogen, and oxygen.
- Find the simplest ratio of the mole values.
- Divide each mole value by the smallest one among them.
- This step ensures the ratio is reduced to the lowest whole numbers.
- Write the empirical formula using the whole-number ratio of atoms.
- In this example, the ratio comes out close to 1 : 2 : 1, so the empirical formula is CH₂O.
In short: Convert percentages → assume 100 g → change to moles → simplify ratio → write formula.