Acids and Bases: Buffers

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Introduction

Molecular Structure

Ionization Constants

Salts

Buffers

Lewis Theory

Buffer Capacity

The following three HONH₂ / HONH₃⁺ buffers all have a base to acid ratio of 1.0. Since they all have the same base to acid ratio, they will all have the same pH; because the base to acid ratio is 1, the pH of each will be the same as the pK_a of HONH₃⁺ (5.96).

Buffer A:		Buffer B:		Buffer C:
	[ HONH₂ ] = 0.10 M		[ HONH₂ ] = 0.50 M		[ HONH₂ ] = 1.0 M
	[ HONH₃⁺ ] = 0.10 M		[ HONH₃⁺ ] = 0.50 M		[ HONH₃⁺ ] = 1.0 M

Suppose 1.0 mL 6.0 M NaOH is added to 100 mL of each buffer. What will be the resulting pH of each buffer?

How do I do these calculations?

Perhaps you should click on the test tube above for help in solving this problem.

Since you have correctly calculated the pH of a buffer, you have probably made a calculation error. Why don't you check your calculation and try again?

The correct answers have been entered for you. Review the calculation for Buffer A by clicking the test tube above. The calculations for Buffers B and C are similar. See if you can work through them and get the correct answers.

Good! This shows that it is not only the ratio that affects a buffer's ability to resist changes in pH. The buffer capacity describes the quantity of acid or base a buffer can accommodate without a significant pH change. In this example, Buffer C experiences the smallest change in pH and thus has the highest buffer capacity. Buffer capacity is a direct result of the concentration of acid and base present in the buffer; the more acid initially present, the more base the buffer can accommodate without a significant pH change. Likewise, the more base initially present, the more acid the buffer can accomodate without a significant pH change.

Suppose instead that 5 mL 6 M NaOH is added to Buffer A. What will be the resulting pH?

Need a hint?

Did you check the hint?

Good! This is an example of exceeding the buffer's capacity . Once this happens, a sharp increase in pH will occur. A buffer's capacity can also be exceeded by adding acid. In this case, once enough acid has been added to consume all of the base initially present in the buffer a sharp decrease in pH will occur. Sometimes a buffer that has reached its capacity is referred to as being "broken".

The correct answer has been entered for you. You can review the calculation by clicking on the test tube above.

This is an example of exceeding the buffer's capacity. Once this happens, a sharp increase in pH will occur. A buffer's capacity can also be exceeded by adding acid. In this case, once enough acid has been added to consume all of the base initially present in the buffer a sharp decrease in pH will occur. Sometimes a buffer that has reached its capacity is referred to as being "broken".