posted by Symone on .
the much used drug cocoaine is an alkaloid. Alkaloids are noted for their bitter taste, and indication of their basic properties. Cocaine, C17H21NO4 is soluble in water tot he extent of 0.17 g/100 ml solution, and a saturated solution has a pH of 10.08. What is the Kb for cocaine?
Let's call cocaine something simple so as to save all that typing, say R3N.
R3N + HOH ==> R3NH^+ + OH^-
Kb = (R3NH^+)(OH^-)/(R3N)
pH = 10.08, solve for pOH from
pH + pOH = pKw = 14
Then calculate OH^- from
pOH = -log(OH^-). That will also be (R3NH^)+.
For R3N, substitute the solubility in M = moles/L.
0.17g/100 mL = 1.7 g/L and 1.7 is how many moles. moles = grams/molar mass
Post your work if you get stuck.
i got 3.2 for my pOH and 6.31x10^-4 for my [OH^-]. after this i am stuck and not sure where to go.
You got what for pOH.
pH + pOH = 14
pOH = 14-pH = 14-10.08 and that isn't 3.2. When you obtain the correct value for OH^-, then substitute that for (OH^-) in the Kb expression. Substitute the same number of (R3NH^+). That leaves something to be substituted for (R3N) in the Kb expression.
The problem tells you the solubility of cocaine is 0.17 g/100 mL. That is the same as 1.7 g/L. How many moles is that?
1.7/molar mass cocaine = moles and that in 1L is M. Substitute that and solve for Kb.
The answer is 8.5x10^-9