You submerge an aluminum block into a large graduated cylinder and notice that the water displaces by 15ml. what is the mass of the aluminum block?

According to the Archimedes Principle, the buoyant force on the submerged block is equal to the weight of displaced water.
Because the buoyant force balances the weight of the block, the weight of displaced water is the weight of the block.

The density of water is approximately 1.0 g/mL.
Therefore the weight of the block is
(15 mL)*(1.0 g/mL) = 15 g.

1.0 g

The answer is: the mass of the aluminum block is 40.5 grams.

V(Al) = 15 mL; mass of aluminium block.

d(Al) = 2.7 g/mL; density of aluminium.

m(Al) = V(Al) · d(Al).

m(Al) = 15 mL · 2.7 g/mL.

m(Al) = 40.5 g; mass of aluminium block.

Density is a measure of mass per unit of volume (in this example grams per milliliter).

No, a chemical reaction is something that changes the chemical compound of a substance.