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## Homework Statement

An ideal gas has a molar mass of 40 g and a density of 1.2 kg m-3 at 80°C. What is its pressure at that temperature?

## Homework Equations

PV=nRT

R constant= 8.314

n= number of moles

T= tempreture in kelvin

density=Mass/ Volume

## The Attempt at a Solution

i simply solved it like this:

i found first the volume:

V=molar mass/ density

V= 0.04kg/ 1.2kg/m^3

= 0.03 m^3

and then i just substituted in the ideal gas law

P=nRT/V (where n is the moles and here we have 1 mole)

P= (1)*(8.314)*(353K)/ (0.03) = 97828.06 Pa

but my professor solved it like this:

T = 273 + 80 = 353 K

PV = nRT

m = rV

so V = 0.04 / 1.2 with n = 1 (since 1 mole of gas has a mass of 40 g, or 0.04 kg)

P = RT/V = 8.3 x 353 x 1.2/0.04 = 8.79x104 Pa

I noticed that he didn't find the volume first and then substituted but he substituted directly in the ideal gas law! why it should be like this!?