Answer:
34.1g of C₅H₅NHCl the student need to dissolve to the solution.
Step-by-step explanation:
Full question is:
A chemistry graduate student is given 500.mL of a 0.20M pyridine C5H5N solution. Pyridine is a weak base with =Kb×1.7x10−9 . What mass of C5H5NHCl should the student dissolve in the C5H5N solution to turn it into a buffer with pH =4.76 ?
Using H-H expression for weak bases, it is possible to find pH of a buffer thus:
pOH = pKb + log [BH⁺] / [B]
Where pKb is -log Kb = 8.77, [BH⁺] concentration of C₅H₅NHCl and [B] concentration of C₅H₅N (It is possible to take the moles of both compounds and not its concentration.
As pH the student wants is 4.76, pOH is:
pOH = 14 - pH = 14 - 4.76 = 9.24
Replacing:
9.24 = 8.77 + log [C₅H₅NHCl] / [C₅H₅N]
Moles of C₅H₅N are:
0.500L × (0.20mol / L) = 0.10mol C₅H₅N
Replacing again:
9.24 = 8.77 + log [C₅H₅NHCl] / [0.10mol]
2.9512 = [C₅H₅NHCl] / [0.10mol]
0.29512 moles = [C₅H₅NHCl].
As molar mass of C₅H₅NHCl is 115.56g/mol, mass of 0.29512 moles are:
0.29512 moles C₅H₅NHCl × (115.56g / mol) =
34.1g of C₅H₅NHCl the student need to dissolve to the solution.