1. Volumetric Calculations How to analyze titration data and perform dilution calculations

2. PGCC CHM 103 Sinex Volumetric procedures Involve the careful measurement of volumes of solutions Involve the careful measurement of volumes of solutions Dissolve materials in water and run reaction in solution - titration in Erlenmeyer flask (designed for swirling to mix reactants) Dissolve materials in water and run reaction in solution - titration in Erlenmeyer flask (designed for swirling to mix reactants) Buret is the measuring device - can read volumes to 0.01 mL Buret is the measuring device - can read volumes to 0.01 mL Pipets may also be used, especially to transfer solutions - good to 0.01 mL Pipets may also be used, especially to transfer solutions - good to 0.01 mL

3. PGCC CHM 103 Sinex Some terminology Analyte Analyte Titrant Titrant Standard solution Standard solution Equivalence point Equivalence point End point End point Substance being analyzed Solution in the buret Solution of known concentration Stoichiometric point

4. PGCC CHM 103 Sinex At the equivalence point For a 1-to-1 reaction such as: For a 1-to-1 reaction such as: HCl + NaOH  NaCl + H 2 O HCl + NaOH  NaCl + H 2 O mole acid = mole base mole acid = mole base moles = molarity x volume in liters moles = molarity x volume in liters So So M acid x V acid = M base x V base

5. PGCC CHM 103 Sinex What is the end point? The end point is the point where an indicator changes color. The end point is the point where an indicator changes color. This is the visual sign to stop or end the titration. If the indicator is selected correctly, the change is at or very near the equivalence point! This is the visual sign to stop or end the titration. If the indicator is selected correctly, the change is at or very near the equivalence point! Example is phenolphthalein Example is phenolphthalein colorless in acid pink in base

6. PGCC CHM 103 Sinex Titrant and analyte In this analysis we determine the volume of a reagent needed to react with our analyte In this analysis we determine the volume of a reagent needed to react with our analyte Principles of Volumetric Analysis Principles of Volumetric Analysis Usually in the form of a Titration Usually in the form of a Titration In a titration increments of a reagent solution called the titrant are added to the In a titration increments of a reagent solution called the titrant are added to the analyte until they have completely reacted. The titrant us usually added from a buret so we analyte until they have completely reacted. The titrant us usually added from a buret so we can determine its exact volume can determine its exact volume

7. PGCC CHM 103 Sinex Good Titrations For a titration two things must occur For a titration two things must occur 1. The K of the reaction must be very large 1. The K of the reaction must be very large 2. The reaction n must go quickly and completely. 2. The reaction n must go quickly and completely. There must be some way to determine when the reaction is complete There must be some way to determine when the reaction is complete

8. PGCC CHM 103 Sinex Good Titrations A. An indicator (or some reagent) can change color A. An indicator (or some reagent) can change color B. A chemical potential may be monitored using a potentiometer (similar to a pH meter) B. A chemical potential may be monitored using a potentiometer (similar to a pH meter) C. The absorbance of the solution may be measured C. The absorbance of the solution may be measured

9. PGCC CHM 103 Sinex Equivalence and end points In determining when the reaction is complete you need to be aware of two terms In determining when the reaction is complete you need to be aware of two terms 1. Equivalence point This is the point at which you have added titrant that is exactly equivalent to the analyte 1. Equivalence point This is the point at which you have added titrant that is exactly equivalent to the analyte 2. End point - This is the point that you observe some change in solution. 2. End point - This is the point that you observe some change in solution. Ideally the Equivalence point and the endpoint are exactly the same, but many times Ideally the Equivalence point and the endpoint are exactly the same, but many times they are a little off from each other. This lead to titration error. It is usually possible to they are a little off from each other. This lead to titration error. It is usually possible to perform a blank titration where the procedure is repeated without an analyte to see howlarge the titration error is. perform a blank titration where the procedure is repeated without an analyte to see howlarge the titration error is.

10. PGCC CHM 103 Sinex Primary,secondary standards and standardization In preparing solution for a titration you need to have a reagent that is of sufficientpurity (>99.9% pure) and stability that it can be quantified directly by weighing. Such material are called Primary standards In preparing solution for a titration you need to have a reagent that is of sufficientpurity (>99.9% pure) and stability that it can be quantified directly by weighing. Such material are called Primary standards Often the titrant we use in a titration is not a primary standard, but is a second reagent which we have made to approximately the correct concentration, and have thenaccurately determined the concentration by comparing it to our primary standard. Thesecond reagent is called a secondary standard Often the titrant we use in a titration is not a primary standard, but is a second reagent which we have made to approximately the correct concentration, and have thenaccurately determined the concentration by comparing it to our primary standard. Thesecond reagent is called a secondary standard The process of calibrating the secondary standard against our primary standard is called a standardization The process of calibrating the secondary standard against our primary standard is called a standardization

11. PGCC CHM 103 Sinex Types of titrations There are two general types of titrations; There are two general types of titrations; direct titrations, where the titrant is added do the analyte until a reaction goes to, and direct titrations, where the titrant is added do the analyte until a reaction goes to, and a back titration, where completion excess reagent is added to the analyte, and then the excess of this added reagent is determined. a back titration, where completion excess reagent is added to the analyte, and then the excess of this added reagent is determined.

12. PGCC CHM 103 Sinex Example of calculation A 25.07 mL sample of vinegar is titrated with 37.31 mL of 0.5119 M NaOH. What is the molarity of the acetic acid (CH3COOH)in vinegar? A 25.07 mL sample of vinegar is titrated with 37.31 mL of 0.5119 M NaOH. What is the molarity of the acetic acid (CH3COOH)in vinegar? CH3COOH+NaOH H2O + CH3COONa CH3COOH+NaOH H2O + CH3COONa HA+NaOH H2O + NaA HA+NaOH H2O + NaA Moles NaOH = 0.5119 moles/L x 37.31 mL x 1L/1000mL Moles acetic acid = moles NaOH = 0.01910 moles M aa = moles/volume = 0.01910 moles/0.02507 L = 0.7618 M

13. PGCC CHM 103 Sinex What is the percent acetic acid in the vinegar? Vinegar has a density near water so our 25.07 mL sample has a mass of 25.07 g. What is the percent acetic acid in the vinegar? Vinegar has a density near water so our 25.07 mL sample has a mass of 25.07 g. Mass of acetic acid = moles x MM = 0.01910 moles x 60.00 g/mole = 1.146 g acetic acid Percent = part x 100/total = 1.146 g acetic acid x 100/ 25.07 g sample = 4.47% acetic acid

14. PGCC CHM 103 Sinex A Back Titration - An indirect analysis for antacids, since they are insoluble An indirect analysis for antacids, since they are insoluble Antacids, which are bases, are reacted with excess acid and this excess acid is titrated with standard base. Antacids, which are bases, are reacted with excess acid and this excess acid is titrated with standard base. Antacid + HCl (initial)  neutralized antacid + HCl (leftover) HCl (leftover) + NaOH  NaCl + H 2 O

15. PGCC CHM 103 Sinex Back Titration Calculation- moles HCl initially = M HCl x V HCl moles HCl initially = M HCl x V HCl moles HCl left-over = moles NaOH moles HCl left-over = moles NaOH = M NaOH x V NaOH = M NaOH x V NaOH mole HCl reacted = mole HCl reacted = moles HCl initially - moles HCl left-over moles antacids - depends on reaction stoichiometry such as: moles antacids - depends on reaction stoichiometry such as: CaCO 3 + 2HCl  CaCl 2 + H 2 O + CO 2

16. PGCC CHM 103 Sinex Dilution - Process of adding solvent, usually water, to lower the concentration of a solution Process of adding solvent, usually water, to lower the concentration of a solution Used to make working standards for spectrophotometry Used to make working standards for spectrophotometry Used to bring a sample into range for analysis Used to bring a sample into range for analysis Typically done using a pipet (TD) snd volumetric flask (TC) Typically done using a pipet (TD) snd volumetric flask (TC) Solute is constant. Solvent is being added.

17. PGCC CHM 103 Sinex Dilution continued Since the solute is constant, then Since the solute is constant, then moles initial = moles final So the dilution equation becomes So the dilution equation becomes C init V init = C final V final where V final = V init + V water

18. PGCC CHM 103 Sinex Dilution Calculation What initial volume of 0.50 M CuSO 4 must be used to make 100 mL of 0.025 M CuSO 4 solution? What initial volume of 0.50 M CuSO 4 must be used to make 100 mL of 0.025 M CuSO 4 solution? C init V init = C final V final and rearrange C init V init = C final V final and rearrange V init = C final V final / C init V init = C final V final / C init =0.025 M x 100 mL/ 0.50 M = 5.0 mL C init V final C final

19. PGCC CHM 103 Sinex Dilution continued A graduated cylinder is a volume measuring device to contain (TC). Could you do a dilution in it? Explain why or why not. A graduated cylinder is a volume measuring device to contain (TC). Could you do a dilution in it? Explain why or why not. Dilution factor = V init /V final Dilution factor = V init /V final 1 mL into 100 mL is a df of 1/100

20. PGCC CHM 103 Sinex Dilution continued Serial dilution – successive dilutions for large scale dilutions in steps Serial dilution – successive dilutions for large scale dilutions in steps A is diluted by 1/100 to B B is diluted by 1/100 to C C is 1/10,000 of A