There are mol of bromide ions in 1.00 l of a 0.300 m solution of albr3.

The equation for molarity of a solution is molarity equals moles divided by liters. Knowing that the solution is .300 M, and there is only one liter, you can use simple algebra to find that there is .3 mols of AlBr3 in the solution. Since moles can be simplified in a chemical equation to a whole number ratio, in this case it would be one mole of Al to three moles of Br. since there are three moles of Br in the compound, all you have to do is multiply the mols of AlBr3 by 3-- equaling to .900 mols of bromide ions.