Question

200c.C. Of a gas measured at s.T.P. Has a mass of 0.268g molecular weight of gas is

Answer 1

pv=nrt n = moles and you have 0.268 g 

1 atm x .2 L = n x 0.0821 x 273 

n = 0.0089 moles 

0.268 g / .0089 moles = 30.11 g/mole 

or, better yet, remember that all gases have 22.4 L / mole at STP. 

you have 0.268 g and 0.2 L 

moles = (1 mole/22.4 L) x ( 0.2 L) Liters cancels out and you get .0089 moles. 

moles = g / molar mass so molar mass = g / moles 

0.268 g / .0089 moles = 30.11 g/mole

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