Manipulating the PV=nRT Equation for Helium Balloon Volume

[brake4country]

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?

 

[Bystander]

legal way

Conditionally. The notation is not what it could be. Subscripts, insertion of masses in the appropriate places would improve things.

PV/n = PV/n (before and after)

It's not obvious that you've done anything without some remarks about initial and final "states" and values.

approx. 9 grams He

Don't overuse the word "approximate" or its derivatives or abbreviations when you've got an exact answer based on the problem statement.