A solution is prepared by dissolving 38.6 g of sucrose (C12H22O11) in 495 g of water. Determine the mole

Question

fraction of sucrose if the final volume of the solution is 508 mL.

Really need help understanding this!

Answer 1

First determine how many moles of sucrose you have. Which is going to be 38.6 g / 342 g per mol = 0.112 mol.

Secondly, determine moles of water. Which is going to be 495 g / 18 g per mol = 27.5 mol.

Since we are looking for mole fraction of sucrose, this will be total sucrose moles divided by total moles, which is 0.122 mol / (0.122 + 27.5 mol) = 0.004.

Volume will not matter in this case because you are given the composition and mass of the solution.  

Answer 2

Please best me if I am going to spend all this time answering General Chemistry Questions.

Moles fractions are just like any other fractions and shouldn't be confusing if you think of them like calculating a test score.  If a test is worth 50 pts (the total) and you score a 38, then your score fraction is the component you got divided by the total, 50--The sum of the points you got (38) and didn't (12).  This leaves us with your score fraction of 38/50.  Also, you didn't get 12 points, so, your non-score fraction is 12 / (38+12) or 12/50.  You will get the analogy as I work through the problem below,

To answer this we need the mole of sucrose (MW=342g/mol), the moles of water (MW=18.0g/mol), and the total number of moles.

Moles of sucrose:

38.6 g sucrose * 1 mol/342 g/mol = 0.113 mol sucrose

Moles of water:

495 g water * 1 mol/18.0 g/mol = 27.5 mol water

Mole fraction of sucrose is the moles of sucrose, 0.113 mol, divided by the total number of moles in this solution or 0.113 mol + 27.5 mol = 27.613 mol.  

Mole fraction of sucrose:

0.113 mol sucrose / (0.113 mol sucrose + 27.5 mol) = 0.113mol/27.613mol

Or

0.00409