A weak base BOH (molar mass 150 g mol ⁻¹ ) is dissolved in 250 g of water. If the degree of dissociation of the base is 30 % and the solution freezes at -0.936^ C , the mass of BOH dissolved is ________ g . (Given: K_f of water is 1.8 K kg mol ⁻¹ )

Step-by-step solution

The degree of dissociation = 30 % = 0.3 . For the weak base BOH B ^+ + OH ^- , the van 't Hoff factor i is given by: i = 1 + = 1 + 0.3 = 1.3 Using the formula for depression in freezing point: T_f = i K_f m 0.936 = 1.3 1.8 m m = 0.936 2.34 = 0.4 mol kg ⁻¹ Molality m = W_ solute M_ solute W_ solvent ( in kg ) 0.4 = W 150 0.25 W = 0.4 37.5 = 15 g Answer: 15