1. Determine the moles of AlBr3:

* Molarity (M) = moles of solute / liters of solution

* Rearrange the formula to solve for moles: moles of solute = Molarity * liters of solution

* moles of AlBr3 = 0.300 M * 0.500 L = 0.150 mol AlBr3

2. Determine the moles of bromide ions (Br-)

* The chemical formula AlBr3 tells us that there are 3 moles of Br- ions for every 1 mole of AlBr3.

* moles of Br- = 0.150 mol AlBr3 * (3 mol Br- / 1 mol AlBr3) = 0.450 mol Br-

Answer: There are 0.450 mol of bromide ions in the solution.