Is that the Chase card? If yes, the following quote frome their terms page seems pretty clear to me. Yes, paying off the whole balance will prevent interest:
How to Avoid Paying Interest on Purchases
Your due date will be a minimum of 21 days after the close of each billing cycle. We will not charge you periodic interest on purchases if you pay your entire balance by the due date each month. We will begin charging interest on balance transfers, cash advances, and overdraft advances on the transaction date.