chemistry
posted by Ryn on .
Prepare 500mL of a 0.200 M acetate buffer at pH 4.90 using only pure acetic acid, 3M NaOH, and water.
1) Add acetic acid to ~400mL of water in a 500 mL beaker. How many grams of acetic acid are needed?
2)Add 3 M NaOH solution until pH is 4.90. What volume of NaOH is needed? (ignore activity coefficients)

You want b/a ratio to be
4.90 = pKa + log(b/a)
I used 4.74 for pKa and came up with b/a = 0.16 approximately or
base = 0.16*acid
base + acid = 0.2
Solve the two equation simultaneously to obtain acid = ??
base = ??
Then you can answer 1). That will be M/2 = moles and moles/molar mass = grams.
2. Follows from 1. base = 0.16 x acid moles.