Colligative properties are directly related to the number of dissolved solute pa
ID: 892479 • Letter: C
Question
Colligative properties are directly related to the number of dissolved solute particles in the solvent. The mathematical equation for the calculation of the freezing point depression of a solution containing a non-volatile solute is Delta Tf = iKfm where Delta Tf is the freezing point depression, K_f is the freezing point depression constant, m is the molality, and i is the van't Hoff factor, which is the effective number of particles in the solution (obtained from the balanced chemical equation). What is i for the sucrose solutions? What is i for the sodium chloride, NaCl solutions? What would i be if Na_2SO_4 was used instead of NaCl?
Explanation / Answer
4. For the depression in freezing point,
dTf = iKfm
a. i is Van't Hoff factor, which is number of ions a substance gives when in solution. Since sucrose does not dissociate, the i for sucrose is 1.
b. NaCl in solution forms Na+ and Cl-. So i for NaCl would be 2.
c. Na2SO4 has i = 3 [2Na+ and 1SO4^2-], whereas, NaCl has i = 1. Now, If i for Na2SO4 is used instead of NaCl, the depression in freezing point which is directly proportional to i, would be greater then when NaCl i is used for calculations.