Question

A 50.0-mL sample of K₂Cr₂O₇ solution oxidizes 1.500 g of Na₂SO₃ to Na₂SO₄. Cr₂(SO₄)₃ is also produced. What is the molarity of the K₂Cr₂O₇ solution?

0.0794 M
		0.476 M
		2.96 M
		0.0952 M
		0.0158 M

Answer 1

answer : 0.0794 M

K₂Cr₂O₇ + 3Na₂SO₃ + 4H₂SO₄ ----------------> Cr₂(SO₄)₃ + 3Na₂SO₄ + K₂SO₄ + 4H₂O

mass of Na2SO3 = 1.500 g

moles of Na2SO3 = 1.500 / 126.04 = 0.0119 mol

from the balanced equation :

1 mol K2Cr2O7   ---------------> 3 mol Na2SO3

     ??                      -------------> 0.0119 mol Na2SO3

moles of k2Cr2O7 = 0.0119 x 1 / 3 = 3.97 x 10^-3 mol

moles = Molarity x volume

3.97 x 10^-3 = Molarity x 0.050

Molarity of K2Cr2O7 = 0.0794 M