We have to use Dalton's law of partial pressures which states that in a mixture of gases each gas will exert its partial pressure corresponding to its molar fraction
In this case mixture consists of hydrogen4moles,helium 3 moles and krypton 2 moles packed in a cylinder of 9 litres and total pressure is 13atm
So partial pressure due to krypton is its mole fraction multiplied by total pressure
As krypton mole fraction is 2/9 its partial pressure is 2/9x13=26/9 atm=2.8888 atm
Now if we introduce 4 moles of xenon into the present system as vol and temperature are fixed
it will result in increase in total pressure corresponding to the moles added.
As the number of moles added are 4,the total moles increases from 9 to 13 and the total pressure in the cylinder will be
pkr= its changed mole fraction xchanged total pressure
=(2/13)x(169/9)=26/9 atm =2.8888atm
so it remains unchanged even if xenon is added
Now if we introduce 2 moles of krypton into the system without any change of temperature and volume
The total moles become 13+2=15 and the total pressure will be 169/9x15/13=195/9 atm
Krypton molar fraction now is 4/15
hence its partial pressure is molar fractionx total pressure
=(4/15)x(195/9)=52/9 atm=5.7777 atm
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