Study Guide on Mole Fraction

Topics: Concentration, Solvent, Water Pages: 3 (693 words) Published: December 4, 2013
Mole Fraction
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The mole fraction is:
moles of target substance divided by total moles involved
The symbol for the mole fraction is the lower-case Greek letter chi, χ. You will often see it with a subscript: χsolute is an example. Example #1: 0.100 mole of NaCl is dissolved into 100.0 grams of pure H2O. What is the mole fraction of NaCl? Solution:

100.0 g / 18.0 g mol¯1 = 5.56 mol of H2O
Add that to the 0.100 mol of NaCl = 5.56 + 0.100 = 5.66 mol total Mole fraction of NaCl = 0.100 mol / 5.66 mol = 0.018
What is the mole fraction of the H2O?
5.56 mol / 5.66 mol = 0.982

By the way, another way to figure out the last substance is 1.00 minus (the total of all other mole fractions). In this case 1.00 - 0.018 = 0.982. Remember that all the mole fractons in the solution should total up to one. Notice that the mole fraction has no units on it and is written as a decimal value. Do not change it to percent. Note of caution: you could see the term "mole percent." It is simply the mole fraction mltiplied by 100. For example, in the problem just below, the mole fraction of cinnamic acid is 0.2885. Its mole percent would be 28.85%. The ChemTeam advises against the use of the tem "mole percent." However, do what your teacher desires you to do.

Example #2: A solution is prepared by mixing 25.0 g of water, H2O, and 25.0 g of ethanol, C2H5OH. Determine the mole fractions of each substance. Solution:
1) Determine the moles of each substance:
H2O ⇒ 25.0 g / 18.0 g/mol = 1.34 mol 
C2H5OH ⇒ 25.0 g / 46.07 g/mol = 0.543 mol
2) Determine mole fractions:
H2O ⇒ 1.34 mol / (1.34 mol + 0.543 mol) = 0.71 
C2H5OH ⇒ 0.543 mol / (1.34 mol + 0.543 mol) = 0.29

Example #3: A solution contains 10.0 g pentane, 10.0 g hexane and 10.0 g benzene. What is the mole fraction of hexane? Solution:
1) You need to determine the moles of pentane, hexane and benzene: to do this, you need the molecular weights. Here are the formulas: pentane: C5H12 
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