Salts split into ions, and every ion counts
- The idea
- Predict
- See it
- Why
- Check
The idea
“0.10 M is 0.10 M, so NaCl counts like glucose and i = 1.”
Molarity is the number you're handed, so it feels like the whole story. Once NaCl dissolves, each unit is two ions.
The short version
The mistake: 0.10 M is 0.10 M, so NaCl counts like glucose and i = 1.
What's actually true: Solute potential depends on how many particles are dissolved. NaCl makes 2 ions, CaCl2 makes 3, and sugars stay whole.
- Solute potential is ψs = −iCRT, where C is molarity. The ionization constant i is how many particles each unit makes when it dissolves.
- NaCl and KCl split into two ions, so i = 2. CaCl2 and MgCl2 make three, so i = 3.
- Glucose, sucrose and urea stay whole, so i = 1.
- For 0.10 M NaCl at 25 °C, ψs = −2 × 0.10 × 0.0831 × 298 = −5.0 bar. With i = 1 you'd get half that.
On the exam: Before you use ψs = −iCRT, ask if the solute splits. NaCl is 2, CaCl2 is 3, and sugars are 1.