The chromium in an aqueous sample was determined by pipetting 10.0 mL of the unknown into each of five 50.0-mL volumetric flasks. Various volumes of a standard containing 12.2 ppm Cr were added to the flasks, following which the solutions were diluted to volume.
Answer to question 1
Data for volume of standard added to 10 mL of unknown sample and corresponding absorbance readings is plotted using spreadsheet as given below:
Slope of line and y intercept are calculated using spread sheet. Slope of line is
Answer to question 2
This is a standard addition experiment. Make a standard addition plot of absorbance versus concentration for mg of standard Cr. The negative of the x-intercept is the mg of the unknown Cr. Plot data pairs (.201, 0 mg Cr), (.292, .010 L 12.2mg Cr L ), etc. The equation for the line is found to be y = 0.7507 x + 0.2042 The x-intercept is –0.2893 so there were 0.2893 mg Cr in the original sample. 0.2893mg Cr 0.010L = 28.9 ppm Cr in the original sample.