Final answer:
The solutions are ordered by their effect on boiling point elevation, accounted for by the van 't Hoff factor, with Zn(NO3)2 having the highest boiling point followed by LiBr, and glucose having the lowest.
Step-by-step explanation:
To order the solutions in terms of decreasing boiling point, one needs to understand colligative properties, specifically boiling point elevation. The boiling point of a solution increases in proportion to the number of dissolved particles (ions or molecules) in the solution.
We need to consider the van 't Hoff factor (i), which corresponds to the number of particles a compound dissociates into when dissolved. Glucose (C6H12O6) does not dissociate and therefore has an i value of 1. LiBr dissociates into two ions, so its i value is 2. Zn(NO3)2 dissociates into three ions (one Zn2+ and two NO3-), giving an i value of 3.
Hence, a 0.050 m solution has a multiplied concentration effect of 0.150 m for Zn(NO3)2 versus 0.100 m for LiBr. The solution with the highest i value will have the greatest elevation in boiling point.
The correct order from highest to lowest boiling point is thus: 0.050 m Zn(NO3)2 > 0.050 m LiBr > 0.120 m glucose.