What is the sack like organ in the frog?

Um you could be a little more specific: color, size according to other organs, and position inside the frog.

But still, I'm going to guess your either talking about the gall bladder or the urinary bladder. Here is what they look like and there purpose.

Gall Bladder:

It is the small green sack underneither the liver (looks like a booger... no joke). It's purpose is t store bile that is created by liver. The bile is secreted in small amounts and used to help digest food when a frog needs it.

Urinary Bladder:

If you look at the anatomy of a frog it would be the lowest organ in the frog. If disecting the frog it will be an empty sack. Its purpose is to store urine.

I hope this helped! ^_^