The Mistake Bank
Class 12 - Chapter 1: Solutions
Colligative properties depend on number. Did you count the ions correctly?
The "i" Factor (Van't Hoff)
Colligative PropertiesScenario: Compare Osmotic Pressure ($\pi$) of $0.1M$ Glucose and $0.1M$ $NaCl$.
Student sees same concentration ($0.1M$).
Answer: "Both have same osmotic pressure."
(Ignores dissociation of salt!)
Check if Solute is Electrolyte!
Glucose (Non-electrolyte): $i = 1$
$NaCl \to Na^+ + Cl^-$ (Electrolyte): $i = 2$
Formula: $\pi = i \cdot C \cdot R \cdot T$
$NaCl$ exerts double the pressure.
Henry's Law Constant ($K_H$)
Solubility of GasesScenario: Gas A has a higher $K_H$ value than Gas B. Which is more soluble?
Student thinks: "Larger constant means more solubility."
Answer: "Gas A is more soluble."
(Wrong proportionality!)
Higher $K_H$ = Lower Solubility!
Formula: $P_{gas} = K_H \times \chi_{gas}$
Rearranging: $\chi_{gas} = P / K_H$
At constant pressure, solubility ($\chi$) is inversely proportional to $K_H$.
Answer: Gas B is more soluble.
Units of R in Osmosis
Calculation ErrorsScenario: Calculate $\pi = iCRT$. Pressure needed in atm.
Student habitually uses the Thermodynamics value of R.
$$ R = 8.314 \text{ J/mol K} $$
(This gives answer in Pascals/1000, not atm!)
Match Pressure Units!
If pressure is in atm, use:
$$ R = 0.0821 \text{ L atm K}^{-1} \text{mol}^{-1} $$
If pressure is in bar, use:
$$ R = 0.083 \text{ L bar K}^{-1} \text{mol}^{-1} $$
Azeotropes vs Deviation
Non-Ideal SolutionsScenario: A solution shows Positive Deviation from Raoult's Law. What kind of Azeotrope does it form?
Student connects "Positive" with "Maximum".
Answer: "Maximum Boiling Azeotrope."
(Opposite logic applies here!)
Positive Deviation = Minimum Boiling!
Positive deviation means vapor pressure is HIGHER than expected.
High Vapor Pressure = Low Boiling Point.
Answer: Minimum Boiling Azeotrope (e.g., Ethanol + Water).
Relative Lowering Formula
Raoult's LawScenario: Calculate molar mass of solute using relative lowering of vapor pressure.
Student uses Molality ($m$) instead of Mole Fraction ($\chi$).
$$ \frac{P^\circ - P}{P^\circ} = \text{molality} $$
(Incorrect formula.)
It equals Mole Fraction of Solute!
$$ \frac{P^\circ - P}{P^\circ} = \chi_{solute} = \frac{n_2}{n_1 + n_2} $$
For very dilute solutions, you can approximate denominator as $n_1$, but be careful with concentrated ones!
Mixing Volumes (Non-Ideal)
PropertiesScenario: You mix 10 mL Ethanol + 10 mL Water. What is the total volume?
Student adds arithmetically.
Answer: "20 mL"
(Ideal solutions add up, non-ideal ones don't!)
$\Delta V_{mix} \neq 0$
Ethanol and Water form H-bonds (stronger interactions than pure components).
Molecules pull closer together ($\Delta V_{mix} < 0$).
Total Volume will be slightly less than 20 mL.
Confess Your Sins!
"A solution is a homogeneous mixture... until you make a heterogeneous mistake."
Did one of these catch you? Or do you have a different horror story from your last exam?
Scroll down to the comments section below and tell us:
No comments:
Post a Comment