Calculate the relative amounts of a weak acid and its conjugate base that must be used to generate a buffer solution of desired pH. A buffer is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid.
It is therefore used to prevent change in the pH of a solution upon addition of another acid or base. The exact ratio of the base to the acid for a desired pH can be determined from the K a value and the Henderson-Hasselbalch equation.
Suppose you wish to prepare a buffer solution to keep the pH at 4. Of the acids listed, the K a value for acetic acid is closest to the desired hydrogen ion concentration. The pK a of acetic acid is.
Therefore, if you add 0. This is due to the change that occurs when another acid or base is added to the buffer. The change is minimized if the concentrations of acid and conjugate base are equal. The more the ratio needs to differ to achieve the desired pH, the less effective the buffer. As a general rule of thumb, the relative amounts of acid and base in a buffer should not differ by more than tenfold. Therefore, it is very important to be able to identify acid and conjugate base pairs.
The conjugate base is the other product, which has had a proton removed. Therefore, it can be used to prevent change in the pH of a solution. Buffer solutions are used as a means of keeping pH at a nearly constant value in a wide variety of situations. The equilibrium expression for a weak acid is given by:. In order to be an effective buffer, the number of moles of the weak acid and its conjugate base must be large compared to the number of moles of strong acid or base that may be added.
The best buffering will occur when the ratio of [HA] to [A - ] is about Buffers are considered to be effective when the ratio is anywhere between and Preparation of Buffers. Buffers can be made by mixing weak acids or bases with salts of their conjugate bases and acids. For example, a buffer could be prepared by mixing 1. The two reactions that would take place are:. The pH of this buffer could be calculated by using the Henderson-Hasselbalch equation, or by working through a reaction table ICE to calculate equilibrium concentrations of the species in the buffer.
We will do both sets of calculations. Initial 1. Equilibrium 1. Using the Henderson-Hasselbalch equation gives:. These cookies help provide information on metrics the number of visitors, bounce rate, traffic source, etc. This cookie is used to identify the repeat visit from a single user.
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Consider the addition of a strong acid such as HCl. But if we keep adding HCl, the weak base A - will eventually run out.
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