What is percent composition by mass of sodium chloride and potassium chloride in the mixture?

Question

A 0.722 g mixture containing only sodium chloride and potassium chloride was dissolved in water. It required 82.1 mL of 0.141 M AgNO3 to completely precipitate all of the chloride present. What is percent composition by mass of sodium chloride and potassium chloride in the mixture?

mass percent NaCl=

mass percent KCl=

Answer 1

moles AgNO3 used = 82.1 ml x 1 L / 1000 ml x 0.141 mol/L = 0.011576 mol AgNO3

moles Cl- present = 0.011576 moles Cl- since Ag⁺ + Cl^- ==> AgCl(s)

Let x = mass of NaCl

0.722 - x = mass of KCl

molar mass NaCl = 58.4 g/mol

molar mass KCl = 74.6 g/mol

mols NaCl + mols KCl = 0.011576

mols NaCl = x g NaCl / 58.4

mols KCl = 0.722 g - x / 74.6

x/58.4 + 0.722-x/74.6 = 0.011576

solve for x (be sure to check the math)

x = 0.0625 g = mass NaCl

0.722 - 0.06 25 = 0.6595 g KCl

mass % NaCl = 0.0625 g / 0.722 g x100% = 8.66% NaCl

mass % KCl = 91.34%