Question

Gram for gram, potassium bromide lowers the melting point of water much more than sucrose . what are at least 2 reason for this? (colligative properties)

Answer 1

Ans :

The colligative properties are dependent on the amount (number of moles) of matter dissolved in the solution.

1) The molar mass of potassium bromide is lower than that of sucrose , so for gram for gram the number of moles of potassium bromide will be higher than the number of moles of sucrose.

So higher the number of moles of compound (here potassium bromide) lower will be the melting point.

2) Now potassium bromide KBr is an ionic compound , it dissociates into its two ions K⁺ and Br^- ions , so its vant hoff factor is 2 whereas sucrose is a covalent compound with vant hoff factor of 1 (as it does not dissociate in water).

So more the vant hoff factor , lower will be the melting point.