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### Course: AP®︎/College Chemistry>Unit 8

Lesson 6: Buffers

# Buffer capacity

The capacity of a buffer to neutralize added acid or base depends on the concentrations of HA and A⁻ in solution. For a given ratio of [HA] to [A⁻], the greater the concentrations, the higher the overall buffer capacity. When [HA] is greater than [A⁻], the capacity is higher for added base than acid. When [A⁻] is greater than [HA], the capacity is higher for added acid than base. Created by Jay.

## Want to join the conversation?

• how would you go about determining the capacity of a basic buffer?
• Buffer capacity is defined as the number of moles of acid or base that have to be added to 1 liter to cause its pH to change by 1 unit.

As a formula this is: B = n/ΔpH, where B is buffer capacity (which is unitless), n is the number of moles of acid or base that was added to the buffer per liter of the buffer, and ΔpH is the pH difference between the initial buffer’s pH and the pH after the addition of acid or base to the buffer.

A larger B value would mean the buffer has the capacity to neutralize larger amounts of acid or base before it no longer functions.

Hope that helps.
• When using the ICF table, why does the conjugate base CH3COO- initially have a concentration of 0.250 and 0.0250? Shouldnt it be 0 on the right side?
(1 vote)
• At , how does CH3COO- gain 0.02 moles? How does OH- lose 0.02 moles? Moles of what? Where do the moles come from?
(1 vote)
• Moles just mean amount of molecules of a particular substance here. Specifically a mole is equal to Avogadro’s number of things, or 6.022 x 10^(23) things. So having 0.02 moles of something means we have 0.02 of 6.022 x 10^(23) of that thing, or 0.02 x (6.022 x 10^(23)) = 1.20 x 10^(22) things. So if substances are gaining or losing moles, then the amount of those substances are increasing or decreasing.

The reaction shown for buffer 1 is an acid-base reaction between acetic acid, CH3COOH, and hydroxide, OH^(-). Acetic acid acts as, well, an acid a donates one of its hydrogens to hydroxide which is acting as a base by accepting that hydrogen. Acetic acid therefore converts to its conjugate base acetate, CH3COO^(-), by losing a hydrogen, and hydroxide converts to its conjugate acid water by gaining a hydrogen. The amount of moles that hydroxide decreases by is exactly the same as the amount of moles acetate increases by because they are related by a 1:1 mole ratio in the balanced chemical equation.

Hope that helps.
(1 vote)
• where sis 4.70 com from when doing ph
(1 vote)
• That's the pKa of acetic acid, or the -log(Ka), which is a measure of how strong an acid is. A pKa is a constant specific to a chemical and can be found in most chemistry textbook's data sections. In a problem/exam situation they're almost guaranteed to provide this information.

Hope that helps.
(1 vote)