Question

Calculate the mole fraction of kbr in a solution made by dissolving 0.35 g KBr in 0.405 L water? (d = 1.00 g/mL).

Answer 1

Answer:

The correct answer is 1.31 x 10⁻⁴ moles

Explanation:

Mole fraction= moles of solute/total moles

The solute is KBr, so we calculate the moles of KBr by dividing the mass (0.35 g) into the molecular weight (MM) of KBr:

MM(KBr) = 39 g/mol + 80 g/mol = 119 g/mol

moles of solute = mass/MM = 0.35 g/(119 g/mol) = 2.94 x 10⁻³ moles

Then, we have to calculate the total number of moles, which is comprised by the moles of solute and the moles of solvent:

total moles = moles of solute + moles of solvent

The solvent is water, so we calculate the moles by dividing the mass into the molecular weight of water:

mass of water = 0.405 L x 1000 mL/1L x 1.00  g/mL = 405 g

MM(H₂O) = (2 x 1 g/mol) + 16 g/mol = 18 g/mol

moles of water = 405 g/(18 g/mol) = 22.5 moles

Thus, the total number of moles is:

total moles = 2.94 x 10⁻³ moles + 22.5 moles = 22.503 moles

Finally, the mole fraction is:

mole fraction = 2.94 x 10⁻³ moles/22.503 moles = 1.31 x 10⁻⁴ moles