Question

How many grams of sulfur (S8) must be dissolved in 100.0 naphthalene (C10H8) to make a 0.12 m solution?

Answer 1

An .12 m solution has .12 moles per kg of solvent. Since you have only 100 g you would have.012 moles.

.012 moles of S8 (MW = 256) weight .012 x 256 = 3.1 g

Answer 2

Molar mass of S8 , m = 8*32 = 256 g / mol
Molality , m = no . of moles of the solute / i Kg of the solvent
Given 0.12 m solution that means 0.12 moles of the solute ,S8 dissolved in 1 Kg of napthalene
So no . of moles of S8 , n = 0.12 mol
Molar mass of S8 is = 256 g / mol
--> Mass of S8 , m' = n * m
= 30.72 g