Problem solving tips
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Solutions

- Tricks and Tips to solve difficult problem easily
1
Tip: Follow these simple tips to find the total vapour pressure over the solution phase from the given data:
Say you are given two liquids and both are volatile. When we mix the two liquids, both the liquids will eventually evaporate and exert a pressure over the solution phase.
To make things easier, we are already given the values of the vapour pressure of the pure components. Plus, we are given the amounts of the two components which have been mixed to form an ideal solution.
Step 1- First of all, we will find the number of moles of each component

For this, we will find the molar mass of each component, and then we can find the number of moles of each component by applying this expression
Here, W is the Weight of the component, and M is Molar mass of the component

Step 2- Once we get the values of number of moles of component 1 and component 2, we can find the mole fraction of component 1 and component 2
Step 3 -Now, go on and apply Raoult's law to find the partial vapour pressure of each component:
(Equation 1)
(Equation 2)
Step 4 -Finally, add equations 1 and 2 to find the total pressure:
Now, let's practice this question to become a pro!
Vapour pressure of chloroform and dichloromethane at are 200 mm Hg and 415 mm Hg respectively. Vapour pressure of the solution obtained by mixing 25.5 g of and 40 g of at the same temperature will be:

[Molecular mass of and molecular mass of ]
A
615.0 mm Hg
B
347.9 mm Hg
C
285.5 mm Hg
D
173.9 mm Hg
2
Tip: I hope by now you have studied all the colligative properties of a solution. One of these properties include elevation in boiling point. Can you solve related questions easily? Yes?
Let's imagine two solutions with the same solvent but different solute particles.
To the first, we have added a given amount of a known solute - it's molecular mass is . And the second contains a given amount of an unknown solute with molecular mass . Additionally, we know the value of boiling point elevation in both the scenarios.
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Now, can you find the value of molecular mass of the unknown solute?
In such questions you need to use the formula very smartly!
There are a few simple steps to follow-
Step 1- First of all, we will find the value of using the values given for Solution 1. For this, we will use the expression-
Remember that the value of is constant for a solvent.
Step 2- Now, we can place the value of in the relevant expression to find the molecular mass of the unknown solute.
Practice the question to get the better clarification.
A solution containing naphthalene (mol. mass ) in of carbon tetrachloride yield a boiling point elevation C while a solution of of an unknown solute in the same weight of the solvent gives a boiling elevation of . Find the molecular mass of the unknown solute.
3
Tip: Let's quickly revise the steps to find the degree of dissociation/association
Step 1 - Write the dissociation/association equation. Say, "a" mole of a solute is added to a solvent to make a solution. Let us assume to be the degree of dissociation/association of the solute in the solvent.
Initially a mole
After dissociation

Step 2-Calculate van't hoff factor using the relation

Step3- Finally, calculate by combining steps 1 and 2
Practice these questions by applying the above steps-

The Van't Hoff factor for 0.1 M La solution is found to be 2.74 the percentage dissociation of the salt is :
A
85%
B
58%
C
65.8%
D
52%
There are KI and sucrose solutions with 0.1 M concentration each. If the osmotic pressures of KI and sucrose solutions are 0.465 bar and 0.245 bas respectively, find the degree of dissociation for KI.
A
0.987
B
0.623
C
0.747
D
0.897