To do this, find the individual masses of each element. Divide each of the masses of each individual masses by the total mass of the compound. Copper has a mass of 63.546 grams. Bromine has a mass of 79.904 grams; however, we multiply 79.904 grams by 2 because there are 2 bromines in the compound. If we divide the masses by the total, and multiply by 100, we get the percent by mass. Hope this helps.
