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# Amount of subtance watch

1. A gas has a density of 1.655gdm^-3 at 323k and 1.01 x 10^5 Pa.Calculate Mr

I know that we have to use

PV =nmr to find moles
and then us moles =mass/Mr
but how we will find Volume or Mass???

We have given a density
1.655 =mass/volume
2. (1)PV = nRT

You know this right?

(2)Moles = mass / mr

Therefore if we substitute (2) into (1)

PV = (Mass / Mr) x R x T

but we know Density = mass / Volume right?

Re-arrange said equation above and substitute for volume and you'll get the equation with variables you want.

Hint:
Spoiler:
Show

Something can be cancelled on both sides of your final equation

3. (Original post by Bulletzone)
(1)PV = nRT

You know this right?

(2)Moles = mass / mr

Therefore if we substitute (2) into (1)

PV = (Mass / Mr) x R x T

but we know Density = mass / Volume right?

Re-arrange said equation above and substitute for volume and you'll get the equation with variables you want.

Hint:
Spoiler:
Show

Something can be cancelled on both sides of your final equation

thanks

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