Experiment 21 Chemistry 1220 136 October 21st 2013 Raffeal Bennett Purpose Procedure Refer to pages 76 77 of General Chemistry 1220 Laboratory Manual by The Ohio State University Department of Chemistry Hayden McNeil Publishing 2013 2014 Sample Calculations See attached sheet Discussion The average molarity of H3PO4 and NaH2PO4 in the unknown was found to be 0 06055 M and 0 0791 M respectively In part A the titration required 11 80 mL of NaOH while part B require 19 50 mL of NaOH The data collected for each part was within 10 mL of each calculation The first indicator bromocresol green solution created green blue appearance at the end point and the second indicator thymol blue solution created a blue appearance at its end point Two different indicators were used because two different acids were present in the unknown Indicators depend on the pH value of the equivalence point and two acids present in the unknown have different pH values at their equivalence points There were a couple of sources of error for this experiment A source of accuracy of measurement when using drop size measurements or equipment only certain to limited significant figures For example the volume in the buret is a value we can be 100 certain of Moreover another source of error could have been using too many drops of the indicator which would only skew the data a little bit They are weak acids and are only used in small amounts to prevent contamination of the sample The pH is given in a range because near the equivalence point the slope of the line of pH vs volume acid base added is large and small change in volume creates a large change in pH which makes the value difficult to be sure of Conclusion In part A of the experiment a titration to the first end point was conducted to find the concentration of H3PO4 using bromocresol green solution indicator In part B a titration to the second end point was conducted to find the concentration of NaH2PO4 using thymol blue solution indicator The average molarity of H3PO4 and NaH2PO4 were found to be 0 06055 M and 0 0791 M respectively Indicators are chosen based of the pH value of the equivalence point The two acids in the unknown had very different K values and therefore require different indicators The first titration removes a proton from H3PO4 making it H2PO4 and the second titration removes another proton resulting in HPO4