Question

1. Strong base is dissolved in 565 mL of 0.400 M weak acid (Ka = 3.85 × 10-5) to make a buffer with a pH of 4.07. Assume that the volume remains constant when the base is added.

a. Calculate the pKa value of the scid and determine the number of moles of acid initially present.

b. When the reaction is complete, what is the concentration ratio of conjugate base to acid?

c. How many moles of strong base were intitially added?

2.What concentration of SO32– is in equilibrium with Ag2SO3(s) and 5.10 × 10-3 M Ag ? The Ksp of Ag2SO3 is 1.5 x 10^-14.

I have tried both of these problems a million times and and the only answer that is working is my pka for the first problem. Please help!!

Answer 1

1)

a)

b)

since you are adding a strong base, the ratio can be calculated from the mass action equation:

The dissociation equation of the weak acid

the ratio is:

remember

c) this is the hard one:

You know the ratio above is after adding the base, so you can assume the moles added of base are gonig to increase the initial concentration of the conjugate base, and so is going to decrease the initial concentration of the acid, it can be written as follows:

being X the moles of base added and Ci the initial moles of acid:

Solving for X:

2)

From the reaction:

you write the mass action equation, remember solids are not shown in the equation:

Solving for SO3: