Hello!
To find the number of moles in the given amount of aluminum, we need to know the mass of one mole of aluminum, and divide that by the given amount of aluminum.
One mole of aluminum is equal to about 27 grams. With that, we can divide it by the given amount of aluminum.
198.5 grams / 27 grams ≈ 7.35185185
According the number of significant figures, there are about 7.4 moles of aluminum in 189.5 grams of aluminum.