# Buffer Solution question (mult. choice).

• RESmonkey
In summary, when dissolved in a liter of water, the buffer mixture of .1 mole Ba(OH)2 and .2 mole HBr will buffer the solution, while the buffer mixture of .3 mole KCl and .3 mole HCl will not.
RESmonkey
New here, hello :)

Which of following mixtures will be a buffer when dissolved in a liter of water?

a. .1 mole of Ba(OH)2 and .2 mole of HBr
b. .3 mole of KCl and .3 mole of HCl
c. .4 mole of NH3 and .4 mole of HCl
d. .2 mole of CH3COOH and .1 mole of NaOH
e. .2 mole of HBr and .1 mole of NaOH

My guess is that it has something to do with being a weak acid/base. Unfortunately, this doesn't make sense, because the answer is D, and D has acetic acid and NaOH, which aren't weak?

it is D

0.1 mol from the 0.2 mole ethanoic acid reacts with 0.1 mol NaOH to give 0.1 mol sodium ethanoate.

0.1 mol ethanoic acid also remains.

the buffer is CH3COOH/CH3COONa

hmmm

Using that, couldn't e also be an answer? It has similar concentrations of an acid and base.

but HBr is a strong acid. it fully dissociates in solution. it is stronger than HCl. there is no equilibirum between H+ and HBr.

Gotcha. So the two compounds have to be weak, and one of them has to be left over in the end?

errmm not really. the acid has to be weak but the salt is a strong electrolyte.

e.g. CH3COOH/CH3COO-Na+

CH3COOH <------> CH3COO- + H+

CH3COO-Na+ ------> CH3COO- + Na+

when you add a small amount of acid, the H+ added will combine with the conjugate base CH3COO- from the fully ionised salt to give CH3COOH. there is no drastic change in pH.

when you add a small amount of alkali, the OH- will combine with H+ from the partially ionised acid to minimise the chnage in pH. now, H+ concentration will also decrease, but since the dissociation of the acid is an equilibrium, according to LCP, the equilibrium will shift to the right, and the H+ will be restored.

this also applies to alkaline buffers. try to guess what happens with NH3/NH4Cl?

Nh3 + H30+ <------> Nh4 + H20 ?

you have to write an equation for each species, you will get a clearer overview...

NH3 + H2O <-----> NH4+ + OH-

NH4Cl -----> NH4+ + Cl-

remeber that NH3 dissociates only partially, but the salt ionises fully.

what happens when you add some acid? or some alkali?

## What is a Buffer Solution?

A buffer solution is a solution that resists changes in pH when small amounts of acid or base are added. It is made up of a weak acid and its conjugate base or a weak base and its conjugate acid.

## Why are Buffer Solutions important?

Buffer solutions are important because they help maintain a stable pH, which is necessary for many biological and chemical processes to occur. They are commonly used in laboratories and industries to control the pH of solutions and prevent drastic changes in acidity or basicity.

## How do you prepare a Buffer Solution?

A buffer solution can be prepared by mixing a weak acid and its conjugate base, or a weak base and its conjugate acid, in specific proportions. The amount of each component used will depend on the desired pH of the buffer solution. It is important to use accurate measurements and mix the components thoroughly to ensure a consistent pH.

## What is the pH range of a Buffer Solution?

The pH range of a buffer solution is dependent on the pKa of the weak acid or base used in the solution. Typically, the pH range is within one unit of the pKa value. For example, a buffer solution made with acetic acid (pKa = 4.76) will have a pH range of approximately 3.76 to 5.76.

## How do Buffer Solutions maintain a stable pH?

Buffer solutions maintain a stable pH by reacting with any added acid or base, either by accepting or donating protons, to maintain the equilibrium between the conjugate acid and base. This process is known as the buffer action and prevents drastic changes in pH even when small amounts of acid or base are added to the solution.

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