To find the empirical formula, we need to determine the relative proportions of lithium and bromine in the compound. We can assume we have 100g of the compound, in which case we have 7.9 g of Li and 92.1 g of Br in the compound.

Convert the mass of each element to moles:

Moles of Li = 7.9 g / 6.94 g/mol = 1.14 mol

Moles of Br = 92.1 g / 79.9 g/mol = 1.15 mol

Divide the moles of each element by the smallest number of moles to obtain the simplest whole number ratio:

Li: 1.14 mol / 1.14 mol = 1

Br: 1.15 mol / 1.14 mol ≈ 1

The empirical formula is therefore LiBr.