CHEM 321 1 a. Sample Problems-KEY Test 2 Select the best answer or fill in the blank. Which statement is FALSE about an EDTA titration of a metal ion. The metal-EDTA complex should be less stable than the metal-indicator complex. The metal-EDTA complex should be more stable than the metal-indicator complex. Many metals cannot be titrated at high pH because they precipitate as hydroxides A metal ion will generally give a better titration endpoint if Kf is high b. Which of the following two component solutions is a good buffer 0.50 M NH3 and 1.00 M NH4Cl 0.05 M acetic acid and 1.00 M sodium acetate 0.5 M HCl and 1.0 M NaCl 0.100 M HPO42- and 0.1 M H3PO4 c. The pH at which the average charge of a polyprotic acid is zero is called the isoelectric point equivalence point endpoint isoionic point d. Draw the principal form of glycine (NH2CH2COOH, pKa1 = 2.3 and pKa2 = 9.2) if it is dissolved in a highly basic solution. NH2CH2COO e. The strongest base that can be used in the titration of an acid in CH3NH2 solution CH2NHf. Which statement is FALSE In a precipitation titration the endpoint is sharpest when the pK sp of the precipitate is small An acid-base indicator is a weak acid or base In all titrations a rapid reaction is desirable An Volhard titration is a type of precipitation titration g. Based on the formation constants for EDTA complexes for the following metal ion, which could be titrated at the lowest pH if the other conditions for all of the titrations are the same. Mg2+ ( log Kf = 8.79 ), Sr2+ (log Kf = 8.73). Ca2+ (log Kf = 10.69 ) h. In this type of titration a known excess of reagent is added to the analyte, and the excess is then titrated with a second standard reagent. displacement blank direct back i. If an acid base indicator has a pKa = 8, then its color change occurs approximately over the pH range of _7_ to __9_. j. What is the pKb for NaF (pKa = 3.17 for HF) ____10.83________ k. Assume that you have solutions and a pH meter. 1.0 M NaH2PO4, 1.0 M Na2HPO4, 1.0 M HCl, 1.0 M NaOH. How could you make a buffer with pH =7. For H3PO4 (pKa1 = 2.1, pKa2 = 7.2 pKa3 = 12.2) Add 1.0 M NaOH to NaH2PO4 until pH =7 Add 1.0 M HCl to NaH2PO4 until pH =7 Add 1.0 M NaOH to Na2HPO4 until pH =7 Add 1.0 M HCl to NaOH until pH =7 1 Sample Problems-KEY CHEM 321 l. Which of the following will produce an acidic solution if dissolved in water? NaF NaBr CH3NH2 Test 2 NH4Cl m. Compound H4A has the following pKa’s 1,2,5,9. What is its principle form at pH = 6 H4A H3A H2A2 HA3 n. What is the approximate pH that results if 10.0 mL of a 0.200 M NaOH solution is added to 20 mL of a 0.200 M NH4Cl solution? ( pKa for NH4+ = 9.25) 4.75 7.00 11.00 9.25 2. How many mL of 0.5 M NaOH must be added a 100 mL of a 1.0 M acetic acid solution in order to make a buffer with a pH of 5.00. Ka = 1.810 for acetic acid. First find the ratio required nbase nbase n 1.78 log base 5.00 4.75 0.25 nacid nacid nacid The moles of weak acid available is found nacid = (0.100L)(1.0 mol/L) = 0.100 mol Next recognize that the moles of weak base will be equal to the moles of strong base added and that the moles of weak acid will be reduced when the strong base is added. nNaOH = 0.0640 nNaOH 1.780.100 nNaOH nbase nNaOH 1.78 2.78nNaOH = 0.178 nacid 0.100 nNaOH 128 mL will be required n 0.0640 mol VNaOH NaOH 0.128 L 1 MNaOH 0.50 mol L pH pK a log 3. Determine the approximate pH of the following solutions or at the point indicated. (pKa1 = 2.148, pKa2 = 7.199, pKa3 = 12.15 for H3PO4) a) 0.025 M NaH2PO4 FIRST INTERMEDIATE FROM pH pK1 pK 2 2.148 7.199 4.67 2 2 b) 0.020 M Na3PO4 (weak base) Kb OH Kw 10 14 12.15 1.41 10 2 K a 10 KbC0 1.41 10 0.020 0.0168 2 2 Sample Problems-KEY CHEM 321 Test 2 c) a solution made by adding 40 mL of 0.1 M NaH2PO4 to 80 mL of 0.1 M Na2HPO4 BUFFER 0.040 L 0.100 molNaH2PO4 0.0040 molNaH2PO4 L mol 0.080 L 0.100 Na2HPO4 L pH pK a log 0.0080 molNa2HPO4 [base] 0.0080 7.20 log 7.50 [acid] 0.0040 4. Draw a rough titration curve for the titration of 50 mL of 0.20 M H2A (pKa1 = 5 and pKa2 = 9) with 0.10 M NaOH. Mark the equivalence points and the buffer regions. Which indicators are appropriate for the equivalence points. 0 mL 50 mL 100 mL 150 mL 200 mL weak acid, H 10 5 0.2 1.41 10 3 pH = 2.85 middle of first buffer region, pH = 5 59 7 first equiv point, intermediate form pH 2 middle of second buffer region, pH = 9 0.2 4 second equiv point, weak base, OH 105 6.32 10 5 pOH=3.3 , pH = 11.7 indicator for first equiv point is p-nitrophenol indicator for second equivalence point is Nitramine 5. A solution is 0.200 M in EDTA and 0.200 M in Ca and is buffered at pH = 9. The formation constant for CaY is 4.9x1010. The fraction of EDTA in the form Y at pH = 9 is 0.054. What is the concentration of free Ca in this solution? init equil Ca2+ 0 x + EDTA 0 x CaY Ca EDTA 2 K ' f 2 0.200 x 8.7 106 9 2.65 10 CaY 0.200 0.200 - x K'f Y4 K f 0.054 4.9 1010 2.65 109 K 'f 0.200 x 0.200 2.65 109 x x x2 [Ca2+] = x = 8.7 x 10-6 3 Sample Problems-KEY CHEM 321 Test 2 6. A 50.00 mL sample containing Fe2+ and Fe3+ required 13.7 mL of 0.0120 M EDTA when titrated at pH =2 and 29.6 mL when titrated at pH = 6. What are the concentrations of Fe2+ and Fe3+ in this sample? CEDTA 0.0120 VEDTA 13.7 mL nEDTA VEDTA CEDTA nFe3 nEDTA nEDTA 1.644 10 nFe3 1.644 10 4 nFe3andFe2 nEDTA mole L nFe2 nFe3andFe2 nFe3 nFe2 1.908 10 CFe2 3.816 10 Vsamp mol nEDTA 3.552 10 4 mol nEDTA VEDTA CEDTA nFe2 4 mol CEDTA 0.0120 VEDTA 29.6 mL CFe2 Vsamp 50 mL mole L 4 mol 3 mol CFe3 L nFe3 CFe3 3.288 10 Vsamp 3 mol L 7. A solution is 0.200 M in EDTA and 0.100 M in Mg and is buffered at pH = 10. What is the concentration of free Mg in this solution? init equil Mg2+ 0 x + EDTA 0.100 0.100 + x MgY Mg EDTA x 0.100 Mg 2 4.29 107 0.100 2 K 'f 2 MgY 0.100 0.100 - x K'f Y4 K f 0.054 108.9 4.29 107 K 'f 0.100 x 0.100 4.29 107 x 0.100 x x 0.100 [Mg2+] = 2.33 x 10-8 4 Sample Problems-KEY CHEM 321 Test 2 8 A 35.00 mL sample containing Ni2+ is treated with 20.00 mL of 0.0200 M EDTA. All of 2+ the Ni is complexed leaving an excess of EDTA. Titration of the excess EDTA required 9.77 mL of 0.0212 M Mg2+ . What is the concentration of Ni2+ in the original solution? L liter Vs 35 mL VEDTA 20 mL CEDTA 0.02 nEDTA VEDTA CEDTA nMg VMg CMg VMg 9.77 mL mol L CMg 0.0212 mol L nEDTA 4 10 4 mol nMg 2.071 10 4 mol nEDTAxs nMg nNi nEDTA nEDTAxs CNi nNi Vs nNi 1.929 10 4 mol CNi 5.51074 10 3 mol L 1 9. A 0.9541 g sample of flour was analyzed by the Kjedhal method. The ammonia liberated by the addition of NaOH was distilled into 25.00 mL of 0.06481 M HCl. The excess HCl was back titrated with 4.12 mL 0f 0.05105 M NaOH. Calculate the percent nitrogen in the flour. 5 Sample Problems-KEY CHEM 321 mass sample 0.9541 gm Test 2 L liter moles of HCl at start_____________________________________ VHCl 25 mL mole L moles HCl VHCl CHCl CHCl 0.06481 moles HCl 1.62025 10 3 mole moles of NaOH titrated___________________________________ mole VNaOH 4.12 mL CNaOH 0.05105 L moles NaOH CNaOH VNaOH 4 moles NaOH 2.10326 10 mole moles of unreacted HCl___________________________________ moles xsHCl moles NaOH moles HClreacted moles HCl moles xsHCl moles xsHCl 2.10326 10 4 mole moles HClreacted 1.40992 10 moles of NH3 liberated ________________________________ moles NH3 moles HClreacted moles NH3 1.40992 10 3 mole moles of N in sample _______________________________ moles N moles NH3 mass of nitrogen ______________________________________ gm 2 mass N 14.007 moles N mass N 1.97488 10 gm mole mass N %N 100 %N 2.06989 mass sample 6 3 mole Sample Problems-KEY CHEM 321 Test 2 9. Answer the following questions based on the figure shown. Assume the acid can be represented by H3A. Distribution Diagram for a Triprotic System 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH a) Give approximate values for pKa1 10-3 b) What are the principal species at pH = 9. Approximately, what is the fraction of each species that is present? 9% A3-, 82% HA2-, 9% H2Ac) What is the pH for a buffer made from an equal molar mixture of HA and H2A? pH = 8 d) Give the approximate value for Kb2 and write the equation that corresponds to this constant. HA2- +H2O H2A- + OH- Kb2 = Kw/Ka2 = 10-6 10. Consider the titration curves shown below. a) Which curve best illustrates pH vs. Vbase for NaH2PO4 titrated with NaOH? B b) What are the values of the pKa’s associated with the substance titrated in curve B. 3,8 c) Which curve is most likely to represent pMg vs. VEDTA for a titration of Mg with EDTA? C d) Which curve best illustrates the titration of a weak base (pKb = 4) with strong acid. 7 F Sample Problems-KEY CHEM 321 A Test 2 14 12 14 10 A B 8 12 10 pX 6 8 pX 4 6 2 4 0 2 0 10 20 30 40 50 0 V 0 50 100 V 150 200 14 16 12 C D 10 14 8 12 pX pX 6 10 4 2 8 0 6 0 10 20 30 40 50 V 4 0 20 40 60 80 V 14 12 E F 10 8 pX 6 4 2 0 0.0 10.0 20.0 30.0 40.0 50.0 V ACID BASE INDICATORS INDICATOR Transition Range(pH) cresol purple Congo red Methyl Red p-nitrophenol Cresol Red Thymolphthalein Alizaren Yellow Nitramine 1.2 - 2.8 3.0 - 5.0 4.8 - 6.0 5.6 - 7.6 7.2 - 8.8 8.3 - 10.5 10.1 - 12.0 10.8 - 13.0 8 pK1 pK 2 2 [base] pH pK a log [acid] pH [H ] K1K 2F K1K w K1 F 100