Question

3.(20%) A 1.000-g steel sample was analyzed for its Cr and Mn content by dissolving the sample in concentrated nitric acid, adding concentrated sulfuric and phosphoric acids, and oxidizing the Cr and Mn with potassium periodate. Final dilution to 250.0 mL was made with 0.5 M H₂SO₄. This solution had absorbances of 0.065 at 440 nm and 0.457 at 545 nm.

a. Using the information below, calculate the percentages of Cr and Mn in the steel samples.

Ion	ε @ 440 nm	ε @ 545 nm
MnO4-	99.3	2.32 x 103
Cr2O72-	412	7.41

b. What is the purpose of adding the concentrated sulfuric and phosphoric acids to the sample solution?

Answer 1

a. let Cx be concentration of MnO4^-

and,

Cy be concentration of Cr2O7^2-

then,

0.065 = 99.3[Cx] + 412[Cy]

0.457 = 2.32 x 10^3[Cx] + 7.41[Cy]

solving the two equations,

[Cx] = 1.97 x 10^-4 M

[Cy] = 1.10 x 10^-4 M

thus,

concentration of Mn = 1.97 x 10^-4 M

and

concentration of Cr = 2 x 1.10 x 10^-4 = 2.20 x 10^-4 M

in the steel sample.

b. Concentrated sulfuric acid is added to dissolve the sample, whereas, phosphoric acid is added to remove rust from steel sample.