[RPG] How to explain to the player that Alchemist’s Fire is safe to carry

Unless the backpack becomes a creature unto itself (via something like animate objects) it does not have a 'maximum' weight. Mostly because going down to that granular a level for random equipment is generally not worth the effort it would take. And, as you, yourself, pointed out, different materials can weigh different amounts, so there is no "standard" definition of how much a cubic foot of material weighs.

Carrying capacity is only affected by magical items, like muleback cords or a bag of holding (which only has an effect in that the things inside it weigh nothing in regards to your carrying capacity), or a masterwork backpack (which raises your effective str score by 1 when calculating carrying capacity). Containers such as barrels, waterskins, backpacks, etc... are more of a convenience for describing how your character is carrying all the random stuff adventurers carry.

As a gm, its far too much work and nowhere near fun enough for me to keep track of the size of everything to make sure it could fit in your various packs as long as it doesn't go over your carrying capacity. Also as a GM, I'd be insane or a giant pain to force my players to keep track of that as well. Saying that, though, its all about what I would deem as reasonable, you could probably throw 1000 coins, some pots and pans, food, and whatnot in a bag and I won't question it, but if you try and fit a 3.5 ft long sword in the bag and not have it be easy to steal, or trying to fit a small cursed item that weighs 1000 lbs in your bag without it ripping I'm gonna question you.

Essentially, its on a GM by GM basis.

Good question!

Looking at Table 6-9 Goods and Services in the Core Rulebook we can see that:

• Flask (empty) weighs 1½ lbs.
• Oil (1 pint flask) weighs 1 lb.
• Acid (flask) weighs 1 lb.
• Alchemists fire (flask) weighs 1 lb.

The logical conclusion is that oil, acid and alchemists fire are all magically lighter than air and make a flask weigh 1/2 lb less than it would otherwise. (I say magically because 1 pint of hydrogen would not give 1/2 lb of buoyancy in air.)

The sensible conclusion is that there is a mistake in the weight of empty flasks. I would suggest working with the value for Oil given that it makes sense and we have both weight and volume - while we do not know the details of "oil" in Pathfinder, a density of 0.8-0.9 is a reasonable assumption. So 1 pint of oil weighs about 0.85 lb and the flask weighs 0.15 lb. This also suggests that the error in the weight of an empty flask is due to a misplaced zero - it has been recorded as 1.5 lb instead of 0.15 lb.

Assume that the same flasks are used for alchemists fire and acid it means that you are delivering about 0.85 lb (13.6 oz) of those substances in each 1 lb flask.