What are the mole fraction and the mass percent of a solution made by dissolving 6.77 g of KI in 0.400 L of water (d = 1.00 g/mL)?

Mole fraction of solute =  1.83×10⁻³

1.66 % by mass

Explanation:

Solute: 6.77 g of KI

Moles of solute: 6.77 g . 1mol / 166 g  = 0.0408 moles

Solvent: water

In order to determine the mass, we use density.

Water density = Water mass / water volume

First we convert the volume of L to mL → 0.4 L . 1000 mL / 1L = 400 mL

1 g/mL = Water mass / 400 mL → Water mass = 400 g

Solvent moles : 400 g . 1 mol / 18g = 22.2 moles

Mole fraction of solute = Moles of solute / Total moles

Total moles = Moles of solute + Moles of solvent

Total moles = 0.0408 mol + 22.2 moles = 22.2408 moles

Mole fraction of solute = 0.0408 mol / 22.2408 moles = 1.83×10⁻³

In order to determine the % mass we need to know the mass of solution

Mass of solvent + mass of solute = Mass of solution

400 g + 6.77 g = 406.77g

Let's make a rule of three:

In 406.77 g of solution we have 6.77 g of solute

In 100 g of solution we may have (100 . 6.77) / 406.77 =  1.66 g of solute