Question
Sat February 09, 2013 By: Arathi M
 

In DUMAS method for estimation of nitrogen, 0.3g of an organic compound gave 50ml diatomic nitrogen. The gas was collected at 300K and 715mm of Hg pressure. Calculate the percentage of diatomic nitrogen in the compound.

Expert Reply
Thu March 07, 2013
a. we know that aqueous tension of water at 300 K is 15mm. 
 
mass of the subs.= .30g.
vol. of N2 = 50 ml.
room temp = 300 K.
VAPOUR PRESSURE OF WATER AT 300 K = 15 mm
actual pressure (dry gas ) = 715-15 = 700 mm Hg.
b. CONVERT THE VOL. AT EXPERIMENTAL CONDITIONS TO VOL AT STP.
 
P1 V1 /T1   =  P2 V2 / T2
700 x 50/ 300  = 760 x V2 / 273 
 
= 41.9 cm3 .
 
c. CONVERT THE VOLUME AT STP INTO MASS .
 
222400cm3 of N2 at STP weighs 28 g.
41.9 cm3 would weigh 
 
28 x 41.9  / 22400    g.
 
 d. % of N2.
 
mass of N2 at STP  / Mass of the substance taken x 100.
 
28 x 41.9  x 100  / 22400  x 0.3      =  17.46
Ask the Expert