Label the balls A through L.

I shall refer any ball but the one with unique weight as a standard weight ball.

Place 4 balls on each side of the balance.

 

There are 3 possible cases.   Click the approbriate balance.



Weighing 1, case 1


Weighing 1, case 2


Weighing 1, case 3


The summary shows all the cases together along with three impossible cases for completeness sake.

You will have to zoom in after clicking the summary button.