Exp 3 Part A pH Titrations Slides

8/2/2023 1 CHEM 111: Part A. Titration and Standardization of a Solution Titration and Standardization Learning Objectives: 1. Prepare a standard solution. 2. Conduct a titration to analyze a solution. 3. Calculate % percent difference between two values. 4. Calculate moles of solution. 5. Calculate molarity of solution. 1 2

8/2/2023 2 Part A. Purpose 3 Molarity Molarity ( ) = M moles solute (mol) liters solution (L) Purpose: The purpose is to determine the exact molarity of a sodium hydroxide (NaOH) solution by standardizing the solution with KHC 8 H 4 O 4 (potassium hydrogen phthalate) known as KHP. K H C 8 H 4 O 4 (aq) + Na OH (aq) → KNaC 8 H 4 O 4 (aq) + H 2 O (l) acid base salt (neutral) water (neutral) Neutralization reaction Part A. Standardization • Label a 250- mL beaker as “NaOH.” The NaOH is the titrant and analyte. • Label a 250- mL Erlenmeyer flask as “KHP.” KHP is an acid that reacts with NaOH. • Fill the 250-mL beaker half full of NaOH solution (approximately 100 mL). 3 4

8/2/2023 4 7 Part A. Preparing KHP Standard 1. Tare a weigh boat and weigh about 0.60 g KHP. Record exact mass. 2. Transfer KHP into a 250-mL Erlenmeyer Flask using deionized water from the wash bottle. 3. Add 30.0 mL of deionized water using a 50-mL graduated cylinder. Add 3 drops of phenolphthalein. Part A. Table 1 Fill in Table 1. with mass of KHP Is this the correct number of significant figures ? Yes! Write down every digit on balance. Trial m KHP (g) V i (mL) V f (mL) Δ V (mL) 1 0.5834 2 3* 4* 7 8

8/2/2023 5 9 Part A. Titration 1. Fill burette up to top calibration mark, 0.00 mL. 2. Add the titrant (NaOH) at a slow dripping speed and swirl the solution. 3. When a faint pink color appears stop the dripping of the solution immediately! Record final volume. The indicator turns pink in basic solutions. After all the acid reacts, one additional drop of NaOH turns the solution basic. Part A. Table 1 Fill in Table 1. initial volume ( V i ), final volume ( Vf ), and net volume ( Δ V ). Is this the correct number of significant figures ? Yes! Record burette volumes with 2 decimal places to the right of the decimal point. Trial m KHP (g) V i (mL) V f (mL) Δ V (mL) 1 0.5834 0.00 16.55 16.55 2 3 9 10

8/2/2023 7 Calculating Moles 13 Moles of KHP Mass of Known (KHP) Molar Mass KHC 8 H 4 O 4 (aq) + NaOH (aq) → KNaC 8 H 4 O 4 (aq) + H 2 O (l) KHP 4 x AW of O 8 x AW of C 5 x AW of H 4 x 15.9994 5 x 1.00794 = = 8 x 12.0107 = = = = 96.0856 g mol -1 5.0397 g mol -1 63.9976 g mol -1 39.0983 g mol -1 = 1 x 39.0983 = 1 x AW of K 204.2212 g mol -1 Calculating Moles 14 Moles of KHP Mass of Known (KHP) Moles of NaOH Molar Mass Molar Ratio Coefficients in balanced chemical equation determine molar ratio. 1 KHC 8 H 4 O 4 (aq) + 1 NaOH (aq) → KNaC 8 H 4 O 4 (aq) + H 2 O (l) KHP 13 14

8/2/2023 8 Part A. Calculate Moles KHC 8 H 4 O 4 (aq) + NaOH (aq) → KNaC 8 H 4 O 4 (aq) + H 2 O (l)             1 mol KHP 1 mol NaOH moles of NaOH = 0.5834 g KHP = 0.00285672 mol NaOH 204.22 g KHP 1 mol KHP KHP Mass of Known (KHP) Molar Mass Molar Ratio Moles of NaOH Part A. Molarity 16 Molarity ( ) = n M V moles solute = liters solution   0.00285672 moles NaOH NaOH = liters of NaOH solution Divide by liters of NaOH solution that reacted! 15 16

8/2/2023 10 19 Part A. Calculate Average (0.1726 M + 0.1720 M) Average (x) = = 0.1723 M 2 Compute the average, ҧ𝑥 , (mean), add up all the individual values ( x 1 , x 2 , x 3 , …) and divide by the total number of values, N , as follows 1 2 3 N x x x x x N + + + = Calculate average of two best trials. 20 Part A. Percent Difference Percent difference represents how close your first experimental value is to the second experimental value . Experimental Value #1 - Experimental Value #2 Percent Difference = x 100 Average Value For example, the first experimental value is 0.1726 M and the second value is 0.1720 M . The average is 0.1723 M . ) (0.1726 M - 0.1720 M Percent Difference = x 100 = 0.34823 % = 0.3% 0.1723 M Note* Percent difference determines precision of your data while percent error determines accuracy of your data. 19 20