Question

A solution of potassium permanganate (KMnO4) was found by titration to be 0.05138 M at 24C. What is the molarity when the lab temperature drops to 16C?

Answer 1

Ans. Given,

                        Molarity of solution at 24.0⁰C = 0.05138 M

# Its assumed that addition of KmNO4 does not affect the specific gravity of water. Also, the mass of solvent (water) remains constant but volume varies with changes with temperature.

# Specific gravity of water at 24.0⁰C = 0.997296 kg/ L

So, Mass of 1.000 L water at 24.0⁰C = 0.997296 kg       

Ref: https://www.simetric.co.uk/si_water.htm

# Specific gravity of water at 24.0⁰C = 0.998943 kg/ L

Now,

            Volume of 0.997296 kg water (= 1 L at 24.0⁰C) at 16.0⁰C = (0.997296 / 0.998943) L

                                                                        = 0.998351 L

So, 1.000 L of solution at 24.0⁰C has a volume of 0.998351 L at 16.0⁰C.

# Now, using             C1V1 = C2V2

C1= Concentration, and V1= volume of initial solution 1         ; soln. at 24.0⁰C

C2= Concentration, and V2 = Volume of final solution 2         ; soln. at 16.0⁰C

Putting the values in above equation-

            0.05138 M x 1.000 L = C2 x 0.998351 L

            Or, C2 = (0.05138 M x 1.000 L) / 0.998351 L = 0.05146 M

Thus, molarity of solution at 16.0⁰C = 0.05146 M