weight gains

if you have a 100 gram snake and feed it a 15 gram mouse when you weight it 2 days later should the snake be weighing 115 grams?

I'm going to go with my gut here and say no.

It could, depends on if it has urinated, how much water it has drank and if it has defacated.

100 + 15 = 115
115 - (poop+pee) = bigger snake

I think your logic is forgetting about the digestive system. Do snakes not burn calories or take nutrients from the food? Or dissolve some of that weight in the stomach acid?

There's no chance a snake will gain 10-15% of its body weight weekly. I'm new to snakes but it just doesn't make sense.

No it will not. To get the most accurate weight i would say weighing them right after they take a dump would be best.

i was thinking this 2 but because they they are so much smaller than human i was not exactly sure

I weigh mine after they shed (sheds are normally accompanied by dump as well) I wouldn't worry too much about getting an exact weight, +/- 5 grams isn't going to matter in the long run