Uv/vis analysis of a cobalt complex at its lambda_max giving a reading that is o

Question

Uv/vis analysis of a cobalt complex at its lambda_max giving a reading that is off scale. A direct calibration prepared from cobalt standards at the same wavelength shows that the method is linear between a low standard with an absorption of 0.10, to a high standard with an absorption of 2.0. The best method of handling of the off-scale sample is to switch to the method of standard additions. measure the sample at different wavelength with a lower molar absorptivity dilute the sample volumetrically until an absorbance reading within the 0.1 - 2.0 range is obtained, prepare a new series of higher-concentration standards Potassium permanganate solutions may be standardized by titrating against primary standard sodium oxal; according to the following equation: 2MnO_4^1- + 5C_2O_4^2- + 16H^+ rightarrow 2Mn^2+ + 10CO_2 + 8H_2O What is the molarity of the KMnO_4 solution if 43.80 mL of the KMnO. is required to titrate 0.2168 grams of Na_2C_2O_4?

Explanation / Answer

Solution :-

Lets first calculate the moles of the Na2C2O4 using its mass and molar mass

Moles = mass / molar mass

Moles of Na2C2O4 = 0.2168 g / 133.9985 g per mol

                                   = 0.001618 mol Na2C2O4

Now using the mole ratio of the C2O4^2- and KMnO4 we can calculate the moles of the KMnO4

0.001618 mol C2O4^2- * 2 mol MnO4^- / 5 mol C2O4^2- = 0.0006472 mol MnO4^-

So moles of the KMnO4 = 0.0006472 mol

Now lets calculate the molarity of the KMnO4

Molarity = moles / volume in liter

                = 0.0006472 mol / 0.04380 L

                = 0.0148 M

So the molarity of the KMnO4 = 0.0148 M

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