Question

1. The Ka of an acid is 2.24x10^-7. You have an acid concentration of 0.120 M and a base concentration of 0.100 M. What is the pH of this buffer?

Report your pH to two decimal places.

2. Buffer capacity refers to the ratio of conjugate base to acid required to make an appropriate pH buffer.

True

False

Answer 1

1.

Let, x ml of 0.120 M acid is mixed with y ml of 0.100 M base. As it is an acidic buffer we have to have x > y.

Now, x ml of 0.120 M acid contains [0.120(x/1000)] mol acid and y ml of 0.100 M base contains [0.100(y/1000)] mol base. Now, since x > y, therefore, the amount of salt produces after the mixing will be [0.100(y/1000)] mol and the remaining amount of acid is [(0.120x -0.100y)/1000].

The total volume of the solution now becomes, (x+y) ml. Therefore, the final concentration of the salt in the buffer solution would be, [0.100y/(x+y)] and the final concentration of the acid would be [(0.120x -0.100y)/(x+y)].

Now, the working formula for calculating the pH of a buffer solution is given by,

- this is known as Henderson equation.

Now, putting the concentration values in the equation we get,

The Ka of the acid is 2.24 X 10^-7. Therefore, pKa = - log₁₀(2.24 X 10^-7) = 6.65

Therefore,

-- this is the general formula for this problem with any arbitrary volume of acid and base with the constraint of x > y.

In this problem, the volumes of acid and base are not mentioned therefore we are assuming that they are added in equal volumes, which means x = y. Therefore, our final equation becomes,

2.

False.

' The ratio of conjugate base to acid required to make an appropriate pH buffer '-- this is a true statement and the ratio is related to the buffer capacity but the 'ratio' itself is not the definition of buffer capacity.

Buffer capacity is defined as the amount (moles) of externally added acids or bases required to change the pH of the buffer solution by unity. mathematically, we can write, buffer capacity(f) as,

where = amount of acid or base added externally

and = change in pH of the buffer solution.