A 50 g piece of aluminum is heated to 100 degree C and then put into a beaker containing 150 ml of water at 20 degree C. Assuming no heat loss to the surroundings, determine the final temperature of water?

Heat give up or gained by a substance is equal to the temperature change (delta T) times its mass in grams times its heat capacity. Since no heat is lost to the universe, the heat gained by the water (X - 20) (150 g)(4.18) plus the heat lost by the Al (X - 100)(0.90)(50 g) is zero. Solve for X and you get 25.36 degrees. In this case I used the joule equivalent of heat capacity.