Will a heavy racket help you hit the ball harder? Power and pace in tennis are generated by the monentum of the racket head at impact.
Generally speaking, the lighter the racket, the quicker you'll swing it through the ball and the faster your shot will travel.
Racket weight has little relation to a player's strength. Pancho Gonzales and Billie Jean King both use a lightish 13-ounce racket.
Check with your tennis professsional or a pro shop to see if your racket is the right weight for you. Don't assume that a heavier racket will add power to your game. A lighter racket more likely will put more pace on your shots.