Acids and Bases: Buffers
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Introduction
Molecular Structure
Ionization Constants
Salts
Buffers
Lewis Theory

How does the buffer from the analysis kit maintain a pH of 10?

The answer lies in the ingredients of the buffer. The buffer is a mixture of ammonia (NH3) and ammonium (NH4+).

Choose the phrase that best completes the following statements:

A solution of NH3 will have a pH

A solution of NH4+ will have a pH

Good! Since ammonia (NH3) is a weak base, it will have a pH above 7 and since ammonium (NH4+) is a weak acid, it will have a pH below 7.

Ammonia (NH3) is a weak base and ammonium (NH4+) is a weak acid.

Ammonia (NH3) is the conjugate base of ammonium (NH4+).

Ammonium (NH4+) is the conjugate acid of ammonia (NH3).





If a small amount of hydrochloric acid (HCl, a strong acid) is added to this buffer solution, what will it react with?

NH3 NH4+

Will an acid (such as HCl) be more likely to react with another acid (NH4+) or a base (NH3)?

Since NH3 is a base, it will react with the added acid:

NH3 + H+ NH4+

Similarly, if a base (for example, sodium hydroxide, NaOH) is added, it will react with the acid in the buffer, NH4+:

NH4+ + OH- NH3 + H2O

This is how a buffer maintains a near constant pH. Every buffer is made up of a conjugate acid-base pair. If an acid is added to the buffer, it is neutralized by the base; if a base is added to the buffer, it is neutralized by the acid.

What is the pH of this buffer solution?

The equilibrium between ammonia (NH3) and ammonium (NH4+) can be described with the chemical equation:

NH4+ + H2O NH3 + H3O+

NOTE: The equilibrium between NH3 and NH4+ can also be described with the chemical equation:

NH3 + H2O NH4++ OH-

It is easier to use the first equation since we are trying to calculate pH (a measurement of the H3O+ concentration).

The buffer is made by dissolving 2.7 g ammonium chloride (NH4Cl) in 17.8 mL concentrated ammonia (NH3, 16 M) and diluting to 100 mL with distilled water.

What are the NH4+and NH3 concentrations in this solution? (You can ignore any proton transfers.)

[ NH4+ ] = M [ NH3 ] = M

Concentrations can be calculated by converting each amount to moles and then dividing by the final volume of the solution (in liters).

While your answer is numerically correct, you have not recorded it to the correct number of significant figures. Try again!

The correct concentrations have been entered for you. You should review these types of calculations in your text.

Click on the Check box again to finish calculating the pH of the buffer.

Good! These concentrations can be used in the acid ionization constant to calculate the pH of the solution:

NH4+ + H2O NH3 + H3O+

Let's look at the general case of a weak acid (HA) and its conjugate base (A-):

If we want to calculate pH, we should isolate [ H3O+ ] on one side of the equation:

Since pH is equal to the negative log of [ H3O+ ], take the negative log of both sides of the equation:

Since log (a x b) = log a + log b, the equation can be rewritten:

Just as the negative log of the hydrogen ion concentration is called pH, the negative log of the acid ionization constant is called pKa. With this substitution and since - log (a / b) = log (b / a), the equation can be written:

This equation is called the Henderson-Hasselbalch equation and is used to calculate the pH of buffers.