Question

Which has the minimum freezing point ?

• A. One molal $NaClaq.$ solution
• B. One molal $CaC{l}_{2}aq.$ solution
• C. One molal $KClaq.$ solution
• D. One molal urea $aq.$ solution

Answer 1

Answer: (B)

Depression in freezing points is colligative property. The solute which produces highest number of ions will have minimum freezing point.

(a) One molal $NaCl$ aqueous solution

$\to N{a}^{+}+C{l}^{-}$

$\therefore$ 2 ions / molecule

(c) one molal KCl aqueous solution

$\to K^{+}+C{l}^{-}$

$\therefore$ 2 ions/ molecule

(d) One molal urea aqueous solution$\to$ no dissociation

(b) One molal $CaC{l}_2$ solution

$CaC{l}_2\to C{a}^{2+}+2C{l}^{-}$

$\therefore 3$ ions/molecule

$\because CaC{l}_2$ solution has highest number of ions

$\therefore$ it has lowest freezing point.