Using your knowledge of equilibrium, pH, Ka and Kb values, choose a buffer pair
ID: 508493 • Letter: U
Question
Using your knowledge of equilibrium, pH, Ka and Kb values, choose a buffer pair from the acids, bases and salts provided in Table 2 (see next page) to prepare two buffer solutions. (one of pH 9.0 and one of pH 7.0) Determine the correct molar ratio of acid or base to salt needed for the selected pH. Then determine the mass (for solids) or volume (for liquids) of the chemicals needed to make 100 mL of the buffer with a Buffering Capacity so that 50 mL of the buffer is capable of neutralizing at least 2 mL of either 1 M HCl or 1 M NaOH without letting the pH change by more that 1 pH unit. See your text Sample Exercise 17.5, pages 733 & 734. You may also need to look up the solubility of each chemical. See the MSDS forms for this information.
Table 2. Available Acid-Base pairs for Preparation of buffer solutions in this experiment Formula Molecular Ka Chemical Type 60.05 1.8 x 10 4.75 Weak acid acetate base of anhydrous above acid weak acid NaH2PO4 H20 137.99 6.2 x 10 a 7.21 Sodium phosphate mono basic Sodium Conjugate NazHPO4 7H20 268.07 4.7 x 10 13 12.32 phosphate, base of above acid weak acid ceHeor .H20 210.14 7.5 x 10 4 3.128 Citric acid monohydrate Sodium citrate Conjugate Na3CeHsOT 2H20 294.11 dihydrate base of above acid 17.3 5.9 x 10 10 9.24 1.7 x 10-5 4.76 NH40H Ammonia 2.0 M Weak base solution Ammonium conjugate NHAC chloride acid of aboveExplanation / Answer
Two buffers are phosphate buffer(containing sodoium hydrogen phosphate and disodium hydrogen phisphate) and anmmonium hydroxide buffer(containing ammonium hydroxide and ammonium chloride);.
For PH 7
PH=PKa+log[salt]/[acid]*
Putting the values we get , 7=-log( 6.2*10^-8)+log[salt]/[acid]
or, 7=7.21+log[salt]/[acid] or [Na2HPO4]/[NaH2PO4]=10^-0.21=0.616
For PH 9
Using Henderson Heselbach equation for base buffer we get POH=PKb+log[salt]/[base]
(14-9)=4.76+log[NH4Cl]/[NH4OH] or [NH4Cl]/[NH4OH]=10^0.24=1.737
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