# Physics

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A sample of helium behaves as an ideal gas as it is heated at constant pressure from 273 K to 373 K. If 15.0 J of work is done by the gas during this process, what is the mass of helium present?

I am not sure what equations to use for this problem....

• Physics -

PV=nRT
p dV + V dp = n R dT
p is constant here so
p dV = n R dT
work = integral p dV = nR dT
p(V2-V1) = n R (T2-T1)

so 15 = n R (T2-T1)

• Physics -

doesnt that give me an answer in moles? ...is there a way to convert moles to mg?

• Physics -

so i got the answer to be .01804 moles

and i know one mole of helium is 4.03 g

do i just multiply the two together? and then divide by 10000 to convert g to mg?

• Physics -

never mind. I made a silly error.

i got .01804 moles...multiplied that times 4.03grams and then multiplied that times 1000 and got 72.7mg.

got it! thanks so much!

• Physics -

what are we solving for in this equation?? 15 = n R (T2-T1)
we know n, we know R, we know t1 and t2...so what do we solve for?

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