fill in the ice table, find the Ka, and the mass of NaCH3COO

Question

sodium acetate is added to an existing 500 ml solution of .200 M CH3COOH to form a buffer with a pH equal to 5.000. let x represent the concentration of CH3COO- in the water after the addition of CH3COONa. fill in the ice table

Answer 1

The mass can't be determined without the pKa/Ka value already known. Nor can the Ka value be determined without knowing the mass of the conjugate base added to solution.

However, if we go with the known pKa, this just takes a couple of steps to find the mass of sodium acetate needed to make the 500 mL buffer solution at pH 5 - which is a common task for chemistry and related subjects - like biology/microbiology buffers..

An ICE table can be applied (periods used to space values in columns)

CH₃COOH --> H⁺ + CH₃COO^-

I...0.200M............ 0 ............x

C...-y .....................+y .......+y

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E...0.200 -y ............y .........x + y

pH can be used to report the value of y and solve for x IF the Ka Value is known.

However, this is a great example of Henderson Hasselbalch buffer expression application - as this is typically an AP/College level prompt rather than a first year high school problem to address (and it's more direct/time friendly).

pH = pKa + log (A/HA)

Do some algebra and we'll get: [HA] * 10 ^(pH-pKa) = [A] = x from above. (I can talk you through that part)

Then substitute and solve and we get: x = 0.364 M CH₃OO^- which is the same as the sodium acetate.

To get back to mass apply molarity (M = mol/L) for moles of sodium acetate and then the molar mass of sodium acetate applied for the grams that were transferred to make the buffer.