AC of the sack = 5. Whether or not objects in the sack break is up to you.
From the SRD:
Objects are easier to hit than creatures because they don't usually move, but many are tough enough to shrug off some damage from each blow. An object's Armor Class is equal to 10 + its size modifier (see Table: Size and Armor Class of Objects) + its Dexterity modifier. An inanimate object has not only a Dexterity of 0 (–5 penalty to AC), but also an additional –2 penalty to its AC. Furthermore, if you take a full-round action to line up a shot, you get an automatic hit with a melee weapon and a +5 bonus on attack rolls with a ranged weapon.
So AC = 10(base) +2(for tiny size) -5(for dex) -2(for inanimate) = 5. And you can take a +5 for spending a full-round action to line up the shot. So unless you get a 1, it is a guaranteed hit as long as you don't have a bigger penalty than your BAB.
As for objects breaking in the sack, that is up to you. It is not covered by the rules, so as a GM it is your job to adjudicate it. An easy house rule would be roll d% to determine what, if anything, breaks.