119. Solving a chemistry problem

Product description

In the polarographic determination of lead(II) using a mercury dropping electrode, a weighed sample of the analyzed sample with a mass m was dissolved in a volumetric flask with a capacity of Vk, a few drops of a gelatin solution were added, and the volume of the solution was brought to the mark with dilute nitric acid used as a supporting electrolyte.

Part of the prepared solution was transferred to a polarographic cell thermostated at 25°C, after blowing through the solution with a stream of nitrogen (to remove dissolved oxygen), a polarogram of the solution was taken and the height hx of the polarographic wave corresponding to the diffusion current of lead was measured. Under similar conditions, polarography of a standard solution with a lead concentration cst was carried out and a polarogram with a hCT wave height was obtained.

From the data below, calculate the mass fraction (%) of lead in the analyzed sample. Write an equation for an electrochemical reaction taking place on a mercury electrode.

Draw a diagram of a polarogram and show how the wave height is found graphically. Name and indicate in the figure the parameter by which the substance is identified by the polarographic method.

Explain why a constant temperature of the solution is maintained in the polarographic cell, why a background electrolyte and gelatin are used, and why oxygen is removed from the solution before polarography.

m, g Vk, ml hx, mm hst, mm cst, mol/l

0.8760 100 167 158 4.00*10-4

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