How do I find the empirical formula for aluminium selenide?

You have to have some data, such as masses, or percent: Empirical means it was measured.

The empirical formula for aluminium selenide is the same as it's molecular formula; Al2Se3.
The empirical formula requires both or all atom quantities to be divided by the same number, so Al9Se3 would become Al3Se, but Al7Se3 would stay the same.

To find the empirical formula for aluminium selenide, you need to have some data that provides information about the ratio of atoms in the compound. This data can be in the form of masses or percentages.

If you have the masses of aluminium and selenium in the compound, you can calculate the empirical formula by following these steps:

1. Convert the given masses of aluminium and selenium to moles by dividing them by their respective atomic masses. The atomic mass of aluminium is 26.98 g/mol, and the atomic mass of selenium is 78.96 g/mol.

2. Determine the ratio of moles of each element in the compound by dividing the number of moles of each element by the smaller number of moles calculated in the previous step.

3. If necessary, multiply the ratios obtained in step 2 by a common factor to ensure that they are whole numbers. This step is required because the empirical formula should have the simplest whole-number ratio of atoms.

4. Write the empirical formula using the determined whole-number ratios. In this case, the empirical formula for aluminium selenide is Al2Se3.

Remember that the empirical formula represents the simplest, whole-number ratio of atoms in a compound. In cases where the empirical formula and the molecular formula are different, it means that the compound exists as multiple unit cells.