How much does fire weigh?

I think it’s hot gas and ash, and the mass depends on the specific contents.

It’s basically a chemical process, so it’s like asking how much evaporation or rust weighs.

Depending on how you define the “fire” I guess the answer would be either 0, or it really depends on what you start with. Burning wood has a different set of products (and flame) than burning magnesium.

Have you seen fire in space? Interesting stuff.

At the end of the reaction roughly 62 grams per mole

Like @funkdaddy said, fire is a process, not an object having mass. You could also ask how much electricity, wind, current, or explosion weighs.

The difference between the weight of the object before and after it burns?