Question

a) A series of solutions, containing cobalt(II) ion were prepared and their absorbance measured at 515 nm. The following values were obtained:

Concentration (g/100 mL)	Absorbance
0.8836	0.8238
0.7084	0.6502
0.4362	0.4054
0.1758	0.1640
0.1064	0.0982
0.0702	0.0630

Plot a calibration line Absorbance versus Concentration. From the graph determine the specific absorptivity. (Plot on Excel. Please explain the steps. Thank you so much in advance)

b) Using the data you obtained from the graph in the previous probleem determine the concentration of cobalt(II) in 1.975 g sample of a substance, which was dissolved in water in a 100 mLvolumetric flask and then diluted to the mark. For the absorbance of the cobalt solution from the flask a value of 0.405 was obtained. What is teh percentage of cobalt in the unknown compound? (Include explanation please.)

Answer 1

a) First we need to plot the absorbance vs concentration graph. For that, keep concentration on x -axis and absorbance on y-axis. After that put linear slope keeping intercept at zero. The plot obtained will be:

The equation of the linear fit is:

y = 0.927 x

This means that slope of the line is 0.927

The slope of the line is specific absorptivity. So, specific absorptivity = 0.927.

b) Absorbance for sample = 0.405

According to Lambert Beer law,

A=ecl

A = absorbance = 0.405

e = specific absorptivity = 0.927

c = concentration = ?

l = path length = 1 cm

So, c = A/el = 0.405/(0.927*1) = 0.4369 g/100 ml

concentration = 0.4369 g/100 ml

Sample taken = 1.975 g in 100 ml

So, 1.975 g of sample contains 0.4369 g of cobalt(II)

Percentage of cobalt in unknown sample = (amount of cobalt/amount of sample)*100

= (0.4369/1.975)*100 = 22.12 %