# CHEMISTRY

posted by .

Strong base is dissolved in 675 ml of 0.200 m weak acid (ka=3.25x10^-5) to make a buffer with a ph of 3.95. Assume that the volume remains constant when the base is added.
HA + OH ---> H2O + A^-

calculate the pka value of the acid and determine the number of moles of acid initially present
-so i got pka=4.488 and 0.135 mol HA

When the reaction is complete , what is the concentration ratio of conjugate base to acid.
i got [A^-]/[HA]=0.2897
bob said it was something around 0.117 but i don't get that ..
3.95-4.488=-0.538
10^-0.538=0.2897

Then it says , How many moles of strong base were initially added?... All i know the strong base has to be less than the number of moles of acid but i have no clue how to get the moles of OH^-.

Can you guys help me? Thank you so much!
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• CHEMISTRY -

I worked this problem for you yesterday, in detail, and explained that 0.135 moles was right and 0.2897 for the ratio was not right. Why not look up your earlier post and you can get all of the details. Also I noted that you have written m. m stands for molality. M stands for molarity. I assumed you meant molarity.