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.) that might be used Consulting the table of indicators in your text, choose an indicator 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 flexi curve. From your graph, determine the concentration of a phosphate sample with an absorbance of 0.496. Submit your graph with the prelab.

Explanation / Answer

A)Molarity of NaOH = 0.1

Volume of NaOH used = 10mL for complete neutralisation

Molarity of acid = ?

Volume of acid used = 10mL

Moles of acid used = 3 X moles of NaOH used

Molarity of acid X volume of acid =3 X molarity of NaOH X volume of NaOH

Molarity = 3 X 0.1 X 10 / 10 = 0.3 M

in the given titration between phosphoric acid and NaOH we will use two indicators

Methyl orange : pH range = 3- 5

it will change colour from red to yellow as pH changes from 3 to 5

Phenolphthlein : pH range = 8-10

It will change colour from colourless to pink whne the pH changes from 8 to 10.

2) The curve will be:

Equation :

y = 6904x + 0.137

x = concentration

y = absrobance

Given

y = 0.496

0.496 = 6904 x + 0.137

x = 5.2 X 10^-5 M = the concentration

Volume of solution concentration Final phosphate concentration Absorbance 0 0.01 0 0 0.5 0.01 0.00005 0.648 1 0.01 0.0001 0.85 1.5 0.01 0.00015 1.18 2 0.01 0.0002 1.46

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