A metal, M , of atomic mass 56 amu reacts with chlorine to form a salt that can be represented as MClx.

Question

A boiling point elevation experiment is performed to determine the subscript 𝑥, and therefore, the formula of the salt. A 39.0 g sample of the salt is dissolved in 100.0 g of water and the boiling point of the solution is found to be 377.88 K. Find the formula of the salt. Assume complete dissociation of the salt in solution.

Answer 1

M = Fe (look it up in the periodic table)

MCl_x will either be FeCl₂ or FeCl₃

But assuming you aren't supposed to look up the unknown metal, we can proceed as follows:

molar mass of unknown = 56 + 35x (since atomic mass Cl = 35)

For the colligative property boiling point elevation, the equation that is used is...

∆T = imK

K = contant = 0.512

∆T = change in boiling point = 377.88K - 373.15K = 4.73º

i = van't Hoff factor = 1 + x (1 for M and x for Cl)

m = molality = moles solute/kg solvent = [39 g/(56 + 35x (0.512) ] ÷ 0.1 kg

This then becomes a math problem, so good luck. Plug in the above information into ∆T = imK and solve for x which should be the number or Cl present in MCl_x