Hi,

 

The Question:

 

For 1dm cubed of a gas in normal conditions with a mass of 2.86g, calculate the average relative molecular mass and density at 17C and 207kPa

 

Please explain to me the method of solving this.

 

My working for the average relative molecular mass:

 

M=mPV/RT

M= 2.86x207x1/8.314x290

M= 0.2455g/mol

Mr=0.2455

 

This however is not correct, as the answer in the textbook is 64.1

 

Thanks.

Edited October 8, 201312 yr by pyroglycerine

The question is worded a little weirdly or you haven't written it down properly. It's asking you two different things under two sets of conditions.

1. 1 dm³ (or 1 L) of gas at STP has a mass of 2.86 g, find its relative molecular mass.

 

2. Calculate the density of that gas at 17^oC and 207kPa

 

Point being, in scenario 1, the pressure and temperature is standard (1 atm and 298 K). You can use PV = nRT or you could simply recall the fact that 1 mole of gas at STP occupies 22.4 L.

Thank you very much for clarifying that for me.

The reason it's worded weirdly it's translated from Chinese and that's also probably the reason I didn't understand the question properly.

Anyway, great help, thanks again!