Question: What is the density (in g/L) of ethyne gas at 743 torr and 12.2°C? The unrounded molar mass of ethyne is 26.036 g/mol.

Question

I did some unit conversions, so 743 torr=0.97763 atm and 12.2°C=285.35°K

I used the equation d=PMr/RT and I plugged in values to get d=(0.97763*23.036)/(0.08205746*285.35)

This gives me 0.96180 g/L. But the HW system I use says that's wrong.

What am I doing wrong? Any help is appreciated.

DrBob222 · Accepted Answer

THE PROBLEM IS THE NUMBER YOU ENTERED. You entered 23.036 and it should be 26.036 if you want to keep all those places. The 1.08 answer is correct.

DrBob222 · Answer

I think you are making one error in substitution and perhaps another in reporting.
First, that's 26.036 and second some data bases won't let you report too many significant figures. Correct the 26 substitution first and if necessary than round to 3 significant figures as dictated by the 743.(and I assume that is 743. and not 743).Let me know that this works out for you.

bobpursley · Answer

checking your units in your final density equaiton
d=(0.97763*23.036)/(0.08205746*285.35)
Units: atm*grams/(mol*liters*atm/mol)
Units: grams/liter
so the units are correct. I dont like taking temp and pressure past three digits, what you began with. That brings d=.962 g/L
that is all I see wrong. It may want the density in other units....

Jamie · Answer

I found an online calculator to check the work, and it gave me 1.086 g/L, which the HW system accepted.

I don't really understand what I did wrong... It seems to depend on how I enter it into my calculator:

(0.97763*23.036)/(0.08205746*285.35) gives me 1.086
(0.97763*23.036) Enter, ANS/(0.08205746*285.35) gives me 0.962

So I don't know. I got the right answer now. I'll ask my professor about it tomorrow.

Thank you for your help!