Mass of water 1.) 98.44g 2.) 97.68g 3.) 99.7g EDTA is .01 M Change in mL from ti
ID: 589664 • Letter: M
Question
Mass of water 1.) 98.44g 2.) 97.68g 3.) 99.7gEDTA is .01 M
Change in mL from titration 1.) 6.08 2.) 5.62 3.) 8.72
Estimate the hardness of water in mg CaCO3/L.
Find %RSD.
Discussion questions uiuis tne Water in Statesboro, GA? Background: In this week's laboratory you will determine the hardness of the local water supply. 'Hardness' of water is based on the properties and behavior of Ca2 and Mg? ions. The analysis is based on a complexometric titration that detects both of these ions using the chelating agent EDTA. The encd point will be observed using the cation-complexing indicator eniochrome black T Complexometric titrations using EDTA are discussed in section Chapter 11 of the textbook. The most important information in the chapter that relates to this lab has also been condensed in the Ch11 lecture notes which have been posted to Folio. If you want a sneak preview of what the titration will look like, you might want to watch this YouTube video Getting ready for this laboratory Complete the L8 prelab. Just like the other prelabs assigned through Folio, your prelab is only available until 15 minutes prior to your laboratory period. As usual, get your lab notebook ready for the new lab. This includes starting on a fresh clean page and i) writing the title, l) writing a concise procedure that will be able to follow, without referral to anything else, ii) getting a table ready for the data that you will generate, and iv) updating your notebook's table of contents. 1. 2. Procedure Perform triplicate titrations according to the following instructions Into each of three clean and dry 250 mL Erlenmeyer flasks add approximately 100 mL of tap water. DO NOT use DI water. Your analysis is of local tap water. Although it is not critical what amount of water you choose to analyze, you need to use a (pan) balance to determine the volume of tap water that you have added to each beaker with the appropriate number of significant figures 1. 2. To each solution that will be titrated add approximately 3.0 mL of pH 10 buffer solution. 3. To each solution add 3-5 drops of eriochrome T indicator to give the solution a distinct purplish-red coloration. 4. Titrate with the EDTA solution until the equivalence point. Again, consult this YouTube video for suggestions on how to perform this titration. The concentration of the titrant EDTA solution will be provided. Before leaving the laboratory for
Explanation / Answer
Estimate hardness of water
Trial 1,
moles EDTA = molarity x volume
= 0.01 M x 6.08 ml = 0.0608 mmol
moles of CaCO3 = 0.0608 mmol
mg of CaCO3 = 0.0608 mmol x 100.086 g/mol = 6.085 mg
hardness in mg/ml CaCO3 = 6.085 mg/0.09844 L = 61.82 mg/L CaCO3
Trial 2,
moles EDTA = molarity x volume
= 0.01 M x 5.62 ml = 0.0562 mmol
moles of CaCO3 = 0.0562 mmol
mg of CaCO3 = 0.0562 mmol x 100.086 g/mol = 5.625 mg
hardness in mg/ml CaCO3 = 5.625 mg/0.09768 L = 57.58 mg/L CaCO3
Trial 3,
moles EDTA = molarity x volume
= 0.01 M x 8.72 ml = 0.0872 mmol
moles of CaCO3 = 0.0872 mmol
mg of CaCO3 = 0.0872 mmol x 100.086 g/mol = 8.727 mg
hardness in mg/ml CaCO3 = 8.727 mg/0.0997 L = 87.54 mg/L CaCO3
Average hardness of water = 68.98 mg/L CaCO3
%RSD = sq.rt.[(x - mean)^2/2] = sq.rt.[(51.2656 + 129.96 + 344.474)/2] = 16.21 mg/L of CaCO3
x = individual trial hardness values
Discussion Q's
1. In the titration pH was adjusted to 10 to ensure all of the metal cation in solution has complexed with added EDTA and remains in soluble state after complexation.
2. If the indicator is complexed more strongly with the cation than with EDTA, this would give error in EDTA titration. Less amount of EDTA would be used in this case than the actual cation present in solution as only some are in free state and able to react with EDTA.
3. The water sample in the experiment is moderately hard.
4. the two cations, Ca2+ and Mg2+ can be analyzed by reacting with oxalate. magnesium oxalate is more soluble and would remain in solution whereas, calcium oxalate which has lower Ksp value would precipitate out first from the solution.