Answer
$0.005738$ mol
Work Step by Step
Aluminum chloride has a molar mass of 133.33. Therefore .2550g of AlCl3 corresponds to $.2550/133.33=.001913$ mol AlCl3. Since there are three moles of chloride atoms for every one mole of AlCl3, the number of moles of cholride ions is $.001913*3=.005738$ mol