Question

A given mass of a gas occuples 919.0 mL in dry state at STP. The same main when collected over water at `15^(@)C` and 750 mm pressure occupies one litre volume. Calculate the vapour pressure of water at `15^(@)C`.

Answer 1

Step 1. To calculate the pressure of the dry gas at `15^(@)C` and 750 mm pressure
`{:("Given Conditions at STP","Final Conditions"),(V_(1)=919 mL,V_(2)=1000 mL),(P_(1)=760 mm,P_(2)=?("Dry state")),(T_(1)=273 K",",T_(2)=273+15=288 K):}`
By applying gas equation, we have `(760xx919)/(273)=(P_(2)xx1000)/(288)" or " P_(2)=(760xx919xx288)/(1000xx273)=736.7mm`
Step 2. To Calculate the vapour pressure of water at `15^(@)C`.
Vapour pressure of water =Pressure of the moist gas-Pressure of the dry gas,
=750-736.7=13.3 mm
Alternatively, if p is the vapour pressure of water at `15^(@)C`, then take `P_(2)`=(750-p)mm.Substituting in the equation, `(P_(1)V_(1))/(T_(2))=(P_(2)V_(2))/(T_(2))`, we get `(760xx919)/(273)=((750-p)xx1000)/(288)`. Solve for p.