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## Homework Statement

A balloon initially contains 20 grams of helium at a pressure of 1000 torr. After some helium is let out of the balloon the new pressure is 900 torr, and the volume is half of what it was. If the temperature has not changed, how much helium is now in the balloon?

## Homework Equations

PV=nRT

## The Attempt at a Solution

I first converted the grams to moles and the pressure to atm. I attempted this problem by setting the first and after values to each other. I used PV/n = PV/n (before and after). When I set these equal to each other I get approx. 4.6 mol He or approx. 9 grams He. Is this a legal way to approach this problem?