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  4. On a humid day in summer, the mole fraction of gaseous H2O (water vapour) in the air at 25° C can be as high as 0.0287 . Assuming a total pressure of 0.977 atm. What is the partial pressure of dry air?

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Q. On a humid day in summer, the mole fraction of gaseous $ {{H}_{2}}O $ (water vapour) in the air at $ 25{}^\circ C $ can be as high as $ 0.0287 $ . Assuming a total pressure of $ 0.977 $ atm. What is the partial pressure of dry air?

KEAMKEAM 2006

Solution:

$ {{P}_{{{H}_{2}}O}}={{X}_{{{H}_{2}}O}}{{P}_{total}} $
$ =0.0287\times 0.977 $ $ =0.028\text{ }atm $
$ {{P}_{total}}={{P}_{dry\,air}}+{{P}_{{{H}_{2}}O}} $
$ {{P}_{dry\,air}}={{P}_{total}}-{{P}_{{{H}_{2}}O}} $
$ =0.977-0.028=0.949\text{ }atm $