Answer:
The correct answer is 1.31 x 10⁻⁴ moles
Explanation:
<em>Mole fraction= moles of solute/total moles</em>
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
