Question 1

What is the Van't Hoff factor?

Accepted Answer

The Van't Hoff factor is the ratio of the number or moles of dissolved particles to the number or moles of solute formula units initially dissolved. It corrects colligative-property expressions for association and dissociation.

Question 2

Why is the Van't Hoff factor 2 for completely dissociated NaCl?

Accepted Answer

Each NaCl formula unit produces one Na+ ion and one Cl- ion. The original one formula unit therefore becomes two dissolved particles, so the particle ratio and i are both 2.

Question 3

How is partial dissociation of an AB electrolyte represented?

Accepted Answer

If alpha is the fraction of AB that dissociates, undissocciated material contributes one particle per formula unit while dissociated material contributes two. The resulting relation is i = 1 + alpha, so partial dissociation gives 1 < i < 2.

Question 4

Why does association give an abnormally high observed molar mass?

Accepted Answer

Association combines two or more formula units into fewer dissolved particles, making i less than 1. Since observed molar mass equals true molar mass divided by i, the measured value becomes higher than the true value.

Question 5

Which expressions include the Van't Hoff factor?

Accepted Answer

The standard forms are Delta T_f = iK_fm, Delta T_b = iK_bm and pi = icRT. Here m and c are the analytical molality and molar concentration before association or dissociation. Relative vapour-pressure lowering follows Raoult's law and is not universally described as simply doubling when i doubles.

Van't Hoff Factor Particle Count 3D Lab

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