1. A microbiologist desires to study the effect of pH on the growth of E. coli b
ID: 542336 • Letter: 1
Question
1. A microbiologist desires to study the effect of pH on the growth of E. coli bacteria. She plans to do the research by preparing culture media that are buffered at three distinctly different pH values. Therefore, she decides to use the triprotic acid phosphoric acid (H3PO), potassium dibydrogen phosphate (KH PO), potassium hydrogen phosphate (K,HPO4) and potassium phosphate (KsPO4) to prepare the buffers = 7.11 x 10-3 K 6.32 x10 K 7.1110- MATERIALS AVAILABLE Standardized solutions: 1.00 MHPO4, 1.00 M KH2PO4;1.00 MK,HPO, 1.00 MK PO, 1.00 MHCI, 1.00 MNaOH Solids: KjPOA, KH PO, K2HPO Volumetric Glassware: Pipets 1.00, 5.00,10.00, 25.00, 50.00 mL, Buret 50.00 mL, Volumetric flask 100.00 ml Analytical balance (weighs to ±0.0001-g)Explanation / Answer
if pH = 12 required, this is nearest to the pKa3, we need thir ionizarion, HPO4-2 and PO4-3
then, if we need [PO4-3] = 0.5 M then...
pH = pKa3 + log(PO4-3/HPO4-)
pKa3 = -log(KA3) = -log(7.11*10^-13)= 12.15
12.00 = 12.15 + log(0.5 / [HPO4-])
10^(12-12.15) = (0.5 / [HPO4-])
[HPO4-] = 0.5 / 0.70794
[HPO4-] = 0.7062 M
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