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## Homework Statement

Calculate the pressure ratio of He to N

_{2}at which helium would have the same density as

*nitrogen*if their temperatures were the same.

## Homework Equations

I used D = m/v

## The Attempt at a Solution

D

_{He}= m

_{He}/v

D

_{N2}= m

_{N2}/v

Both gases occupy the same volume, so just v for both.

Since D

_{He}= D

_{N2},

m

_{Hr}/v = m

_{N2}/v and m

_{He}= m

_{N2}

For some x and y,

x mol He(4.003 g/mol He) = m

_{He}

y mol N

_{2}(28.01 g/mol N

_{2}) = m

_{N2}

4.003x g = 28.01y g

x = 7y

To me it looks like there are 7 times as many moles of He as N

_{2}but I doubt that would directly apply to their pressure ratios. I think I'd have to use PV = nRT but I'm not sure how I'd put it in.

I was actually helping some chemistry students earlier today with this and am hoping I can have the answer ready for them tomorrow morning.

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