The absorbances of four standard iodide solutions were measured spectrophotometr

Question

The absorbances of four standard iodide solutions were measured spectrophotometrically in a 1.00 cm cuvet at 456 nm after the iodide was converted into elemental iodine. The Beer-Lambert plot showing the relationship between the absorbances and the iodide molarities is given below. The equation of this calibration line is y = 188x + 0.0541. A solution containing iodide ions treated with KNO_2 and HCI is placed in a 1.00 cm wide cuvette and inserted into the spectrometer, producing an absorbance reading of 0.430 at a wavelength of 456 nm. What is the equilibrium concentration of the iodide ions in the solution?

Explanation / Answer

Ans. Given, absorbance, y of the sample = 0.430

Trendline equation of the graph is “Y = 188 X + 0.0541 ” in form of “Y = mX + C”

In the graph, Y-axis indicates absorbance and X-axis depicts concentration. That is, according to the trendline (linear regression) equation Y = 188 X + 0.0541 obtained from the graph, 1 absorbance unit (1 Y = Y) is equal to 188 units on X-axis (concentration) plus 0.0541.

Putting Y = 0.430 in trendline equation-

            Y = 188 X + 0.0541

Or, 0.430 = 188 X + 0.0541

Or, 188 X = 0.430 – 0.0541 = 0.3759

Or, = 0.3759 / 188

Hence, X = 0.001999468 = 1.99 x 10-3

Unit of concertation is M.

Thus, concentration of the unknown = 0.00199 M

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