# chemistry help? don't know how to start ?

A metal, M of atomic mass 56 amu reacts with chlorine to form a salt that can be represented as MClx. A boiling point elevation experiment is performed to determine the subscript 𝑥, and therefore, the formula of the salt. A 28.3 g sample of the salt is dissolved in 100.0 g of water and the boiling point of the solution is found to be 376.58 K. Find the formula of the salt. Assume complete dissociation of the salt in solution.

### 1 Answer

- Roger the MoleLv 71 month ago
(376.58 K) - (100.00 + 273.15) K = 3.43°C change

(3.43°C) / (0.512°C/m) = 6.699 m ions

(6.699 mol ions/kg) × (0.1000 kg) = 0.6699 mol ions

When 1 mole of MClx dissolves completely, (1 + x) moles of ions are formed:

MClx → M{x+} + x Cl{-}

(0.6699 mol ions) × ( (1 mol MClx) / (1 + x) mol ions) = 0.6699 / (1 + x) mol MClx

(28.3 g) / (0.6699 / (1 + x) mol MClx) = (42.2451 x + 42.2451) g MClx/mol

(42.2451 x + 42.2451 g MClx/mol) - (56 g M/mol) = (42.2451 x -13.7549) g Clx/mol

The molecular weight of Clx is also equal to (35.4532 x) g/mol. Set the two values for the molecular weight of Clx equal to each other:

(42.2451 x -13.7549) = 35.4532 x

Solve for x algebraically:

x = 2.0252 ≈ 2

So the formula for the salt is MCl2, or, using "56 amu", FeCl2.

(There are shorter ways to get this answer, but they depend on a certain amount of guessing.)