Question

The molal boiling-point constant for water is 0.51 degrees C/molal. The boiling point of a 1.00 molal solution of Ca(NO3)2 should be increased by:

A.) exactly 1.53
or B.) Somewhat less than 1.53

This is how I worked the problem:

Ca(NO3)2 dislocates into 3 mole particles. Therefore, 3 x .51 x 1 molal = 1.53 elevation of boiling point. However, my teacher said the answer was B.) somewhat less than 1.53. What step would I be missing?

Answer 1

The van't Hoff factor of 3 which you have used is correct; however, in REAL terms, the three ions don't act as three but as somewhere between two and three. You may want to read about activity coefficients.