Answer:
4.22% NaBr
Step-by-step explanation:
To find the mole fraction of NaBr, you need to (1) determine the mass of the solute (NaBr) and the solvent (H₂O), then (2) convert the mass of the solute and solvent to moles (via the molar masses), and then (3) calculate the mole fraction of NaBr. The final answer should have 3 sig figs to match the sig figs of the given value (20.1%). Because you are given a percent less than 100%, you can assume that the percent (20.1%) is equal to the mass of the solute (20.1 g).
(Step 1)
100% - 20.1 % NaBr = 79.9% H₂O
20.1% NaBr = 20.1 g NaBr
79.9% H₂O = 79.9 g H₂O
(Step 2)
Molar Mass (NaBr): 102.89 g/mol
20.1 g NaBr 1 mole
------------------- x --------------------- = 0.195 mole NaBr
102.89 g
Molar Mass (H₂O): 18.016 g/mol
79.9 g H₂O 1 mole
------------------ x ------------------- = 4.43 moles H₂O
18.016 g
(Step 3)
moles of solute
Mole Fraction = ---------------------------------------------------- x 100%
moles of solute + moles of solvent
0.195 mole NaBr
Mole Fraction = -------------------------------------------------------- x 100%
(0.195 mole NaBr) + (4.43 moles H₂O)
Mole Fraction = 4.22% NaBr