To determine the molar fraction of a substance, you need first to calculate the number of moles of this substance and all other substances contained in the mixture (solution), and then substitute these values into the following formula:X = n1/Σn , where X is the mole fraction of the substances, n1 the number of moles, and Σn is the sum of the number of moles of all the substances.
Let's consider an example. You face the following problem: a mixture of 29 grams of sodium chloride and 33.3 grams of calcium chloride. It was dissolved in 540 grams of water. You must calculate the mole fraction of sodium chloride in the resulting solution.
First of all, write the formulas of all substances, then, determine their molar masses using the periodic table, which shows the atomic mass of all elements:NaCl – molar mass is 58. Since the atomic weight of sodium is 23 and chlorine– 35 (23+35=58);СaCl2 – molar mass equal to 110. The atomic weight of calcium is 40, chlorine– 35 (40+(35+35))=110);H2O molar mass is 18. The atomic weight of hydrogen 1, oxygen– 16 (1+1+16=18).
To simplify the calculations, you can also round off the values. If desirable high accuracy, then you need the calculations to take into account that the atomic weight of calcium – 40,08, chlorine 35,45, and sodium – at 22.98.
Determine the number of moles of each starting material. To do this, divide the known quantity of sodium chloride, calcium chloride and water in their molar mass and receive the following results:- chloride of sodium: 29 / 58 = to 0.5 mole for calcium chloride: 33,3 / 111 = 0,3 mole;- for water: 540 / 18 = 30 moles.
Substitute all the above values into the formula and determine the molar proportion of sodium chloride. The formula will look as follows:0,5 / (0,5 + 0,3 + 30) = 0,5 / 30,8 = 0,0162.