At 300 K, the vapour pressure of a solution containing 1 mole of n-hexane and 3 moles of n-heptane is 550 mm of Hg. At the same temperature, if one more mole of n-heptane is added to this solution, th

Question

At 300 K, the vapour pressure of a solution containing 1 mole of n-hexane and 3 moles of n-heptane is 550 mm of Hg. At the same temperature, if one more mole of n-heptane is added to this solution, the vapour pressure of the solution increases by 10 mm of Hg. What is the vapour pressure in mmHg of n-heptane in its pure state ?

Canvas Classes · Accepted Answer

Strategy:
 Set up two equations using the total pressure of the solution (Raoult's law) for the two different mixing scenarios. Solve these simultaneous equations to find the pure state vapour pressure of heptane.

Step 1: Set up Equation 1 (Initial mixture)
- Total moles = 
- ,

Step 2: Set up Equation 2 (After adding 1 mol heptane)
- Total moles = 
- , 
- New pressure =

Step 3: Solve the equations
Multiply (Eq 1) by 4 and (Eq 2) by 5:
- 
- 
Subtracting the first from the second:
.

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