Mole Fraction of Solutions - Online Tutor, Practice Problems & Exam Prep

• Mole fraction is represented by the variable capital X.
• Mole fraction represents moles of solute per moles of solution.
• The mole fraction formula is X = moles of solute / moles of solution.
• The formula is used to calculate the mole fraction of a compound, substance, mixture or solution.
• The mole fraction is a dimensionless quantity.
• The mole fraction helps in understanding the concentration of a solution.
• It is used in chemical calculations to determine reactions and properties of a solution.
• The mole fraction is a useful tool in understanding colligative properties.
• Mole fraction can be used to compare the solubility of two substances.
• The mole fraction is an important concept in chemistry and is widely used in various fields of study.

Mole Fraction Concept 1

Mole fraction is represented by our variable capital X and it represents moles of solute per moles of solution. So if we were to take a look at the mole fraction formula, we're just going to say again, mole fraction which is represented by capital X equals moles of solute divided by moles of solution.

So keep this in mind anytime you're faced with any question dealing with the mole fraction of a compound, a substance, a mixture or solution.

X = moles of solute moles of solution

Mole Fraction Example 1

A solution contains eight 5.2 grams of ascorbic acid, which has a molecular formula of C₆H₈O₆. It's a water soluble vitamin and it's in 270 grams of water. Here we need to calculate the mole fraction of ascorbic acid in this solution. All right, so we're going to say here X of ascorbic acid, C₆H₈O₆, would equal the moles of ascorbic acid divided by the moles of solution.

Now remember a solution is composed of solute and solvent together. So really we could transform the bottom to say, moles of our solute plus moles of our solvent, which in this case is water. We have here 85.2g of ascorbic acid. We're going to sit here for every one mole of ascorbic acid. Oops. Which has six carbons, 8 hydrogens, and six oxygen. The combined mass is 176.124 grams, so here grams cancel out and I'll have the moles of ascorbic acid as 0.48375 moles.

So here we're going to plug in 0.48375 moles of ascorbic acid on the top and here on the bottom, plus we the moles of water. So here we have 270 grams of water which is H₂O. The mass of water from the two hydrogens and the one oxygen is 18.016g according to our periodic table. And that's for every one mole of water. Grams cancel out and I'll have the moles of water which come out to be 14.9867 moles.

So take those moles and plug them into our formula. Now what's going to happen is we're going to have moles on the top and on the bottom, and they're going to cancel one another out. So that means mole fraction is a unitless value. So here when we plug that into our calculator, we get back 0.0313 as my mole fraction. Here my answer has three sig figs because from our question 85.2 has three sig figs and 270.0 has four sig figs.

Calculate mole fraction of a 2.4 m aqueous solution of citric acid (C₆H₈O₇).

• A. 0.96
• B. 0.041
• C. 0.55
• D. 0.024

If mole fraction of urea is 4.55 x 10^-1, what is the mass of urea needed to dissolve in 38.0 g of water? The molar mass of urea is 60.062 g/mol.

• A. 26.7 g
• B. 106 g
• C. 39.6 g
• D. 8.19 g