Question

Mole fraction to percent by mass. Convert 0.1538 mole fraction of propanol (C₃H₈O) in water to percent by mass of propanol in the solution. (Some of the calculations from the previous question can speed up the process here.) Answer to 4 significant figures with appropriate units.

Parts per million to molarity. The daily intake of iron is 18 mg for an adult. This dose dissolved as Fe²⁺ ions in 1 liter of water makes an 18 ppm Fe²⁺solution. Convert 18 ppm Fe²⁺ to molarity. Assume the aqueous solution is dilute enough to use the shortcut and the solution density is essentially

equal to water (1.0 g/mL). Answer to 2 significant figures with appropriate units.

Molality to percent by mass. Convert the molality of a 2.35 mol/kg RbNO₃(aq) solution to percent by mass. (Hint: begin by separating the 2.35 mol RbNO₃ and the 1 kg water.) Answer to 3 significant figures with appropriate units.

Answer 1

propanol mole fraction = 0.1538 /1

this means = 0.1538 moles

water moles = 1 - 0.1538 = 0.8462

mass of propanol = moles x molar mass = 0.1538 x 60 = 9.23 g

mass of water = 0.8462 x 18 = 15.23

mass % of propanol = (9.23 / 9.23 + 15.23 ) x 100

                             = 37.73 %

Parts per million to molarity.:

18 ppm = 18 mg /L = 18 x 10^-3 g / L

               = (18 x 10^-3 / 55.85) mol /L

               = 3.2 x 10^-4 M

molarity = 3.2 x 10^-4 M

Molality to percent by mass:

moles of solute = 2.35

mass of solute = moles x molar mass = 2.35 x 147.5 = 346.6 g

mass of solvent = 1 kg = 1000 g

mass % = (346.6 / 346.6 + 1000) x 100

                = 25.7 %