A 0.1 molal solution of a monobasic acid is 45% ionized. Calculate the freezing point depression.

Given:
Molality of the acid (m) = 0.1 molal
Degree of ionization (α) = 45% = 0.45
For water, Kf = 1.86 K kg/mol

Since the acid is monobasic, it dissociates as follows:
HA <=> H+ + A-

The van't Hoff factor (i) is given by:
i = 1 + α(n - 1)
where n is the number of ions produced per molecule of the acid. In this case, n = 2 (1 H+ and 1 A-).

i = 1 + 0.45(2 - 1) = 1 + 0.45 = 1.45

The freezing point depression (ΔTf) is given by:
ΔTf = i * Kf * m
ΔTf = 1.45 * 1.86 K kg/mol * 0.1 mol/kg
ΔTf = 0.2697 K
ΔTf ≈ 0.269 °C

Therefore, the freezing point depression is approximately 0.269 °C. The correct option is 0.269oC