Mass Percent and Molarity - Chemistry 280

Solvent

The majority component of a solution

Solute

The minority component of a solution

A concentrated phosphoric acid solution is 85.5% H3PO4 by mass and has a density of 1.69 g/mL at 25°C. What is the molarity of H3PO4?

The molarity of 85.5% H3PO4 is 14.7

What is the mole fraction of urea, CO(NH2)2, in a solution prepared by dissolving 5.6 g of urea in 30.1 g of methanol, CH3OH?

The mole fraction of CO(NH2)2 is 0.090. For this problem, 5.6 g urea, CO(NH2)2, is the solute and 30.1 g methanol, CH3OH, is the solvent. To find the mole fraction of urea you first have to change grams of both the solute and the solvent to moles by dividing them by their molar masses: 5.6 g CO(NH2)2 / 60.07 g/mol CO(NH2)2 = 0.093 mol CO(NH2)2 30.1 g CH3OH / 32.05 g/mol CH3OH = 0.939 mol CH3OH Next, add the moles of the solute and moles of the solvent to get the total moles of the solution: 0.093 mol CO(NH2)2 + 0.939 mol CH3OH = 1.032 mol solution To find the mole fraction of urea, divide mol solute by total mol solution: 0.093 mol CO(NH2)2 / 1.032 mol solution = 0.090