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Are my calculations correct for this question?


bamdavis

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Must find the molar mass of an unknown volatile liquid.

 

Using the equation PV=mRT

M

 

P= 102.4 KPA

V=0.098L

T= 373 K

Mass= 0.1647 g

R= constant gas for KPA= 8.312 KPA

 

Rearrange it to M= mRT

PV

 

i got M= (0.1647)(8.312)(373)

(0.098)(102.4)

 

M= 50.8841 g/mol

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this is just from a lab i did today.

we put 5ml of an unknown volatile in a volumetric flask, put tin foil on the top and poked a hole in it. We heated it in a beaker filled with boiling water to get it to evaporate. After it evaporated we cooled it to condense into a liquid and then weighed it.

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it doesn't have to be an element, riogho.

 

I havent checked bamdavis's math is correct, but, assuming he is, a mass of 50.9 g/mol is perfectly feasible. it's possible that it's chloromethane (which has a mass of 50.5 ish), for instance... although that's a fairly uncommon solvent for a school lab... i suspect it's not that, but there are many others it could be, especially if you account for the fairly wide range of possible experimental error

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I thought R had to be .08206

 

R is 0.08206 [math]L.atm.mol^{-1} K^{-1}[/math]

 

it's ALSO 8.3145 [math]L.kPa.mol^{-1}K^{-1}[/math]

 

if you put the units into the calculation above you'll see that the second version is the appropriate one to use.

 

There also other values for R. For instance you could have the units [math]L.mmHg.mol^{-1}K^{-1}[/math]

 

or you could have the units [math]cm^{3}atm.mol^{-1} K^{-1}[/math]

 

etc etc etc

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