Now let's see what happens when we add a small amount of strong acid, such as HCl. The Cl - is the conjugate base of a strong acid so is inert and doesn't affect pH, and we can just ignore it.
In fact, we already discussed what happens. The equation is:. In this case, the capacity of the buffer will have been exceeded - a situation one tries to avoid. Now, if we add 0. In the process, the 0. Thus, our buffer did what it should - it resisted the change in pH, dropping only from 3.
What is a buffer composed of? How does a buffer work? These two reactions can continue to alternate back and forth with little pH change. Selecting proper components for desired pH Buffers function best when the pK a of the conjugate weak acid used is close to the desired working range of the buffer.
Example 1: HF Buffer In this example we will continue to use the hydrofluoric acid buffer. Over to Wikipedia …. A conjugate base is what remains after an acid has provided a proton during a chemical reaction. The pH is the ratio between these two types of ion. Buffer solutions do have thresholds. If enough strong acid or base is added to the solution, the pH will ultimately change. This is known as the buffer capacity. Many chemical reactions need a constant pH to work properly, so buffer solutions have many uses.
Industrially, buffer solutions are used in:. Buffer solutions also occur naturally. A buffer is a solution that can maintain a nearly constant pH if it is diluted, or if relatively small amounts of strong acids or bases are added.
A buffer solution can be made by mixing a weak acid with one of its salts OR mixing a weak base with one of its salts. A more technical way of saying this is that a buffer solution consists of a mixture of a weak acid and its conjugate base OR a weak base and its conjugate acid. Which of the following set of solutions could be used to prepare a buffer solution?
In any solution containing a weak acid, there is an equilibrium between the un-ionised acid and its ions. So for ethanoic acid, you have the equilibrium:. The presence of the ethanoate ions from the sodium ethanoate will have moved the equilibrium to the left, but the equilibrium still exists.
Where you have done calculations using this equation previously with a weak acid, you will have assumed that the concentrations of the hydrogen ions and ethanoate ions were the same.
Every molecule of ethanoic acid that splits up gives one of each sort of ion. If the equilibrium has been pushed even further to the left, the number of ethanoate ions coming from the ethanoic acid will be completely negligible compared to those from the sodium ethanoate.
We therefore assume that the ethanoate ion concentration is the same as the concentration of the sodium ethanoate - in this case, 0. In a weak acid calculation, we normally assume that so little of the acid has ionised that the concentration of the acid at equilibrium is the same as the concentration of the acid we used.
That is even more true now that the equilibrium has been moved even further to the left. Now, if we know the value for K a , we can calculate the hydrogen ion concentration and therefore the pH. Remember that we want to calculate the pH of a buffer solution containing 0. You will still have the value for the hydrogen ion concentration on your calculator, so press the log button and ignore the negative sign to allow for the minus sign in the pH expression.
You should get an answer of 5. You can't be more accurate than this, because your concentrations were only given to two figures. Note: I commented further up the page that if you had a very weak acid and one of its salts, the buffer solution formed could well be alkaline. HCN is a very weak acid with a K a of 4. If you had a solution containing an equal numbers of moles of HCN and NaCN, you could calculate exactly as above that this buffer solution would have a pH of 9.
This isn't something that you need to worry about. Just don't assume that every combination of weak acid and one of its salts will necessarily produce a buffer solution with a pH less than 7. You could, of course, be asked to reverse this and calculate in what proportions you would have to mix ethanoic acid and sodium ethanoate to get a buffer solution of some desired pH. It is no more difficult than the calculation we have just looked at. Suppose you wanted a buffer with a pH of 4.
If you un-log this to find the hydrogen ion concentration you need, you will find it is 3. All this means is that to get a solution of pH 4. All that matters is that ratio. One way of getting this, for example, would be to mix together 10 cm 3 of 1. And there are all sorts of other possibilities. Note: If your maths isn't very good, these examples can look a bit scary, but in fact they aren't. Go through the calculations line by line, and make sure that you can see exactly what is happening in each line - where the numbers are coming from, and why they are where they are.
Then go away and practise similar questions. If you are good at maths and can't see why anyone should think this is difficult, then feel very fortunate.
Most people aren't so lucky! We are talking here about a mixture of a weak base and one of its salts - for example, a solution containing ammonia and ammonium chloride.
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