The titration curve shown below is for the titration of phosphoric acid with 0.1

Question

The titration curve shown below is for the titration of phosphoric acid with 0.100 M NaOH. If 10.0 mL of the acid was used, what is its molarity? (You must do a calculation for EACH endpoint individually.) Consulting the table of indicators in your text, choose an indicator that might be used to mark each end point. A series of dilutions was made using a 0.0100 M phosphate standard solution, and the absorbance of each solution was measured. Using a computer program, plot absorbance vs. concentration to obtain the 'standard curve'. Do include 0, 0 as a point, since there will be zero absorbance at zero concentration. Decide whether the data are best represented by a curve or a straight line. If a straight line, let the computer draw the regression line. If a curve, do not have the computer draw the line; draw it yourself using a flexicurve. From your graph, determine the concentration of a phosphate sample with an absorbance of 0.496. Submit your graph with the prelab.

Explanation / Answer

For the first end point volume of acid = 10mL, volume of base = 25 mL

molarity of the acid = 25mL *0.1M/10mL = 0.25 M

For the seconf end point volume of acid = 10mL, volume of base used = 50-25 = 25 mL

molarity of acid = 25mL*0.1M/10 = 0.25 M

Molarity of the acid is = 0.25 M

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The first end point is near pH = 4, methyl orange can be used for this end point. Second end point is near 8. Phnoplthalein indicator can be used to view this end point.

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