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Question

for science class i need to know

how to define how much gas is in an object

Answer 1

A very vague question but I will attempt to decipher.  

You could "define how much gas is in an object" by either the total mass of the gas or by it's mole count. (these two quantities are interchangeable if we know the molecular mass of a given gas)

If these quantities are unknown they could be determined by information about the container, most notably volume and temperature.  Along with pressure measurements.

All of these quantities are related to each other by the Ideal gas law:

PV = nRT

P = pressure

V = volume

n = moles of gas

T = temperature

R = universal gas constant, which varies with units