Osmosis is the net motion of a solvent toward a region of lower solvent concentr

Question

Osmosis is the net motion of a solvent toward a region of lower solvent concentration from a region of higher solvent concentration. Or otherwise stated: the net movement of a solute from a region of higher solute concentration to a region of lower solute concentration. The important concept is that nature tries to disperse the solutes in a heterogeneous (liquid or gas phase) solution to produce homogeneous solution. This leads to a pressure that propels the components of the mixture. The equation used to describe this pressure is similar to the ideal gas law: V = inRT where n, R, T, and V have the same meanings as in the ideal gas law and is the osmotic pressure and i is the van't Hoff factor. What is the osmotic pressure of 2.71 g of NaCl in 1.00 L of water at 20.0 °C?

Explanation / Answer

Let us take the given equation V = inRT

= i x(n/V)xRT = i x M x RT

Let us Convert grams to moles:

2.71 g/L ÷ 58.443 g/mol = 0.0464 mol/L

Now, plug into the above equation:

= (i) (0.0464 mol/L) (0.08206 L atm / mol K) (293 K)

Here i is van 't Hoff factor. What is its value for NaCl?

When NaCl ionizes in solution it produces Na+ ions and Cl¯ ions. One mole of NaCl produces 1 mole of each type of ion. So the van 't Hoff factor is, theoretically, equal to 2. However, we will use 1.8.

[Reason: This has to do with a concept called ion pairing. In solution, a certain number of Na+ ions and Cl¯ ions will randomly come together and form NaCl ion pairs. This reduces the total number of particles in solution, hereby reducing the van 't Hoff factor.]

So, plug again and then solve:

= (1.8) (0.0464 mol/L) (0.08206 L atm / mol K) (295 K)

    = 2.0218 atm

= 2 atm

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