October 4, 2015

Homework Help: Chemistry

Posted by Katherine on Sunday, May 13, 2012 at 4:03am.

I keep getting the last part of this question wrong.

Strong base is dissolved in 765 mL of 0.200 M weak acid (Ka = 4.63 x 10^-5) to make a buffer with a pH of 4.18. Assume that the volume remains constant when the base is added. HA(aq) + OH-(aq) -. H2O(l) + A-(aq). I found that the pKa of the acid = 4.33 and the number of moles of acid initially present is 0.153 mol HA. [A-]/[HA] = 0.71 which are correct but i can't find how many moles of strong base were initially added!

(x/0.153-x) = .71 ----> 0.128 mol of OH is incorrect

and 0.153 [which i really thought would be correct] is incorrect as well!
y = 0.71(0.153 mol - y)
y = 0.153 mol

PLEASE help me!! this problem is frustrating me

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