8. (a) Which conjugate acid-base pair is the best choice to [ make a buffer at p

Question

8. (a) Which conjugate acid-base pair is the best choice to [ make a buffer at pH 7.4 according to the Ka table on CH.COOH the right? Acid Ka 1.8× 1 7.1×1 6.3 × 10 CalliN:04s 3.3x10 C4HI2NO CoHIONO S 4.0x10 Best choice: acid formula and base formula (b) If a buffer contains equal volumes of 0.75 M CaH2NO,Cl and 0.45 M C.HnNOs, what is the pH of the buffer? 9. A 25 mL 0.50 M NH3 solution is added into a 20 mL 0.30 M HCI solution. The Kb of NH3 is 1.8 x 10. What is the final pH of the mixture? 10. The ionization of adenine (CsHsNs), a nucleobase, is listed below. Kai -7.08 x 10- Ka2- 1.58 x 10-10 For a 0.50 M adenine hydrochloride aqueous solution (CsH&NsCL;, completely ionized to CsH&Ns; and Cr in water), calculate [CsH&Ns;]. ICI], [H O']. ICsHNs]. ICSHaNs1, and [OH].

Explanation / Answer

Q8

pH = pKa + log(A-/HA)

we need A-:HA ratio nearest to 1 for a buffer so

pH = approx pKA

pH = 7.4

then

pKa = 7.4 approx

best answer is

Ka = 10^-7.4

Ka = 3.981*10^-8

from th elist, choose  

3.3*10^-8; which is C8H18N2O4S

acid = C8H18N2O4S

base = C8H17N2O4S-

b)

if equal volumes are present, finfd pH

pH = pKa + log(C4H12NO3 / C4H12NO3+)

pKa = -log(8.7*10^-9) =8.06

pH = 8.06 + log(0.45/0.75)

pH = 7.838

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