Calculating the pH of a Buffer

Because conjugate acid—base pairs share a common ion, we can use the same procedures to calculate the pH of a buffer that we used to treat the common-ion effect in Sample Exercise 17.1. Alternatively, we can take an approach based on an equation derived from Equation 17.5. Taking the negative logarithm of both sides of Equation 17.5, we have 

Equation 17.9 is known as the Henderson-Hasselbalch equation. Biologists, biochemists, and others who work frequently with buffers often use this equation to calculate the pH of buffers. In doing equilibrium calculations, we have seen that we can normally n ect the amounts of the acid and base of the buffer that ionize. Therefore, we can usually use the starting concentrations of the acid and base components of the buffer directly in Equation 17.9. 

Analyze We are asked to calculate the pH of a buffer containing lactic acid (HC3H5O3) and its conjugate base, the lactate ion (C3H503 ). 

Solve The initial and equilibrium concentrations of the species involved in this equilibrium are 

Because K is small and a common ion is present, we expect x to be small relative to either 0.12 or 0.10 M. Thus, our equation can be simplified to give 

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As in Example 6.13, the Henderson-Hasselbalch equation provides a simple way to calculate the pH of a buffer and to determine the change in pH upon adding a strong acid or strong base. 

Any solution containing comparable amounts of a weak acid, HA, and its conjugate weak base, A-, is a buffer. As we learned in Chapter 6, we can calculate the pH of a buffer using the Henderson-Hasselbalch equation. 

Buffers are solutions that resist a change in pH when we add an acid or base. A buffer contains both a weak acid (HA) and its conjugate base (A-). The acid part will neutralize any base added and the base part of the buffer will neutralize any acid added to the solution. We may calculate the hydronium ion concentration of a buffer by rearranging the Ka expression to yield the Henderson-Hasselbalch equation, which we can use to calculate the pH of a buffer ... 

The common-ion effect is an application of Le Chatelicr s principle to equilibrium systems of slightly soluble salts. A buffer is a solution that resists a change in pH if we add an acid or base. We can calculate the pH of a buffer using the Henderson-Hasselbalch equation. We use titrations to determine the concentration of an acid or base solution. We can represent solubility equilibria by the solubility product constant expression, Ksp. We can use the concepts associated with weak acids and bases to calculate the pH at any point during a titration. 

Taking the negative log of both sides yields the Henderson—Hasselbalch equation, which can be used to calculate the pH of a buffer ... 

Use the Henderson-Hasselbalch equation to calculate the pH of a buffer solution that is 0.25 M in formic acid (HC02H) and 0.50 M in sodium formate (HC02Na). 

Calculating the pH of a Buffer 248 Dilution of Buffer Solutions 251 Buffer Capacity 251 Alpha Plots 252... 

Calculate the pH of a buffer solution containing 0.05 M acetic acid and 0.1 M sodium acetate. The pKa of acetic acid is 4.74. The ionic strength of the solution is 0.10 M. 

This equation is used to directly calculate the pH of a buffer without using an equilibrium calculation. It is important that you know how to do equilibrium calculations before you use this equation. The equation makes the calculation much faster, but there are frequently conceptual questions about acid-base equilibria that require a solid understanding of the topic. Therefore, make sure you truly understand what you are doing before using this... 

Equation 14.10 is known as the Henderson-Hasselbalch equation and is provided for you on the AP formula sheet. You can use it to calculate the pH of a buffer. There is a similar form of the equation that is rewritten to address the hydroxide ion concentration... 

Calculating the pH of a Buffer Solution Following the Addition of a Strong Acid or Strong Base. 

On the AP exam, acid-base questions appear on a regular basis, and buffer calculations are among the most common of those questions to pop up. One of the things you are expected to be able to do is calculate the pH of a buffer solution, and though you may not realize it yet, you have already done this calculation. In the previous example of the common ion effect, when you calculated the pH of the solution made with the 0.30 M acetic acid and the 0.30 M sodium acetate, that was a buffer solution. That is the first type of buffer calculation you are expected to know. The second type of calculation is to determine the effect of adding a strong acid or base to a buffer, and the... 

Calculate the pH of a buffer solution in which both the acetic acid and sodium acetate concentrations are 1.00 X 10 M. The equilibrium constant, Xa, for acetic acid is 1.75 X 10 . 

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