Solutions
Easy Overview
Imagine you're making lemonade. Too much sugar and it's syrup. Too little and it's sad. Solutions are about what dissolves in what, and how much. This chapter is basically the math behind mixing things.
Concentration expressions
There are so many ways to say 'how much stuff is in there'. Mass percent, mole fraction, molarity, molality. Molarity changes with temperature (volume changes). Molality doesn't (mass stays same). Board exams love asking the difference.
Henry's law
The gas dissolved in a liquid depends on its pressure above the liquid. Open a Coke bottle ā pressure drops, gas escapes. That's Henry's law in real life. Deep-sea divers get this problem too ā nitrogen dissolves in blood under pressure, then bubbles out if they surface too fast.
Raoult's law
The vapor pressure of a solution depends on how much of each component is there. Like a team project ā each member contributes based on how many of them there are. Ideal solutions follow this perfectly. Real solutions? Not so much.
Colligative properties
These depend only on HOW MANY particles you add, not WHAT they are. Add salt to water ā boiling point goes up (elevation), freezing point goes down (depression). That's why we salt roads in winter. Also why your car radiator needs antifreeze.
Abnormal molar masses
Sometimes the molar mass calculated from colligative properties is weird ā too high or too low. That's because particles are associating (clumping together) or dissociating (splitting apart). NaCl splits into Na+ and Cl- ā twice the particles, so freezing point drops twice as much.
Key Points
- ā¢Molarity = moles / volume (L), changes with temperature
- ā¢Molality = moles / mass (kg), temperature independent
- ā¢Henry's law: p = KH Ć x (pressure proportional to mole fraction)
- ā¢Raoult's law: p = pĀ° Ć mole fraction of solvent
- ā¢Four colligative properties: VPL, BPE, FPD, OP
- ā¢Van't Hoff factor (i) accounts for association/dissociation
- ā¢i > 1 for dissociation, i < 1 for association
Practice Questions
- Calculate the boiling point elevation when 10g of glucose is dissolved in 200g of water.
- Distinguish between ideal and non-ideal solutions with examples.
- A 0.5 M solution of KCl shows freezing point depression twice that of urea. Explain.