hello, i totally dunno how to do this qn..
N-butylamine, CH3(CH2)3NH2 is a weak mono-acidic organic base. A chemist starts with a 25cm^3 solution containing 2.00 x 10^-3 mol N-butylamine and he wishes to make a buffer solution of pH 10.00 using N-butylamine and its salt, CH3(CH2)3NH3+Cl-
a) Given that the pKa value of N-butylammonium ion, CH3(CH2)NH3+, is 10.61 at 25ºC, calculate the pH of N-butylamine solution. (ans: 11.76)
b) An equal volume of aqueous N-butylamine salt is added to the N-butylammine solution to produce a buffer solution of pH 10.00. Calculate tej concentration of the N-butylammine salt in the buffer solution. (ans: 0.163moldm^-3)
thanks a lot :)
a)
pKa = 10.61
pKb = 14 - 10.61 = 3.39
Kb = 10^(-3.39)
[OH-] = sqrt(Kb*conc. of N-butylamine)
= sqrt(10^(-3.39) * 2*10^-3 / 0.025)
pOH = 2.243455007
pH = 14 - 2.243455007 = 11.76 (to 2 dp)
b)
pOH = pKb + lg ( [salt]/[base] )
14 - 10 = 3.39 + lg( [salt]/(2*10^-3 / 0.050) )
[salt] = 0.163 mol dm^-3 (to 3 sf)