The correct answer is C. In the titration of a weak acid with a strong base, the conjugate base of the weak acid will make the pH at the equivalence point greater than 7. Therefore, you would want an indicator to change in that pH range. Both methyl orange and bromocresol green change color in an acidic pH range, while phenolphtalein changes in a basic pH.
Privacy Policy. Skip to main content. Acid-Base Equilibria. Search for:. Learning Objectives Calculate the concentration of an unknown strong acid given the amount of base necessary to titrate it. Key Takeaways Key Points An acid — base titration is used to determine the unknown concentration of an acid or base by neutralizing it with an acid or base of known concentration. Neutralization is the reaction between an acid and a base, producing a salt and a neutralized base.
A strong acid yields a weak conjugate base A — , so a strong acid is also described as an acid whose conjugate base is a much weaker base than water. Common examples of strong bases are the hydroxides of alkali metals and alkaline earth metals, such as NaOH and Ca OH 2.
Very strong bases are even able to deprotonate very weakly acidic C—H groups in the absence of water. Example: What is the unknown concentration of a Learning Objectives Distinguish a weak acid-strong base titration from other types of titrations. Key Takeaways Key Points In an acid — base titration, the titration curve reflects the strengths of the corresponding acid and base.
If one reagent is a weak acid or base and the other is a strong acid or base, the titration curve is irregular, and the pH shifts less with small additions of titrant near the equivalence point.
Acid-base titrations depend on the neutralization between an acid and a base when mixed in solution. Key Terms stoichiometry : The study and calculation of quantitative measurable relationships of the reactants and products in chemical reactions chemical equations.
Learning Objectives Recall that strong acid-weak base titrations can be performed with either serving as the titrant. Key Takeaways Key Points The titration is typically performed as an acid into base. Following the titration with a pH meter in real time generates a curve showing the equivalence point.
If the approximate pH of the equivalence point is known, a colorimetric indicator can be used in the titration. Key Terms buffer : A solution used to stabilize the pH acidity of a liquid. Polyprotic Acid Titrations Polyprotic acids, also known as polybasic acids, are able to donate more than one proton per acid molecule.
Learning Objectives Recall the general shape of a pH vs equivalents graph generated by titrating a polyprotic acid. Although the subsequent loss of each sequential hydrogen ion is increasingly less favorable, all of the conjugate bases are present in solution. Key Terms titration : The determination of the concentration of some substance in a solution by slowly adding measured amounts of some other substance normally using a burette until a reaction is shown to be complete—for instance, by the color change of an indicator.
Acid-Base Indicators An indicator is a weak acid or a weak base that has different colors in its dissociated and undissociated states. Learning Objectives Explain which, of a given series, would be the best acid-base indicator for a given titration. You need to be careful with the descriptions of a titration: the chemical with unknown concentration is being titrated by the titer, which is the chemical with known concentration.
The equivalence point in a strong acid and strong base titration, whichever way around, should occur at pH 7. A second horizontal phase with a levelling-off, like the beginning, after the equivalence point. The equivalence point in these titrations will occur at a pH greater than 7 basic conditions. Another near horizontal level-off after the equivalence point where very little pH change occurs.
The equivalence point in these titrations will occur at a pH lower than 7 acidic conditions. Another near horizontal level-off after the equivalence point where very little pH change a very slow drop occurs.
Titrating a strong acid with a strong base needs an indicator with a range covering pH 7, for example bromothymol blue which has a pH range of 6. This ensures the equivalence point where a massive change in pH occurs with minimal titer being added will be flagged by the color.
Titrating a weak acid with a strong base means the equivalence point will be at a pH above 7, so pick an indicator with a pH range above 7. Phenolphthalein is a very good choice for titrating acids with strong bases as it has a pH range from 8. Titrating a weak base with a strong acid means your equivalence point will be at a pH below 7, so you need an indicator with a pH range below 7 too.
Methyl red is a good choice with a pH range of 6. Using titration data in graphs. How do graphs explain titration? Notice that the equivalence point is now somewhat acidic a bit less than pH 5 , because pure ammonium chloride isn't neutral. However, the equivalence point still falls on the steepest bit of the curve. That will turn out to be important in choosing a suitable indicator for the titration. At the beginning of this titration, you have an excess of hydrochloric acid.
The shape of the curve will be the same as when you had an excess of acid at the start of a titration running sodium hydroxide solution into the acid. A buffer solution is formed containing excess ammonia and ammonium chloride. This resists any large increase in pH - not that you would expect a very large increase anyway, because ammonia is only a weak base. For the first part of the graph, you have an excess of sodium hydroxide. The curve will be exactly the same as when you add hydrochloric acid to sodium hydroxide.
Once the acid is in excess, there will be a difference. Past the equivalence point you have a buffer solution containing sodium ethanoate and ethanoic acid. This resists any large fall in pH. The start of the graph shows a relatively rapid rise in pH but this slows down as a buffer solution containing ethanoic acid and sodium ethanoate is produced.
Beyond the equivalence point when the sodium hydroxide is in excess the curve is just the same as that end of the HCl - NaOH graph. It so happens that these two are both about equally weak - in that case, the equivalence point is approximately pH 7. This is really just a combination of graphs you have already seen. Up to the equivalence point it is similar to the ammonia - HCl case. After the equivalence point it is like the end of the ethanoic acid - NaOH curve.
Notice that there isn't any steep bit on this graph. Instead, there is just what is known as a "point of inflexion". That lack of a steep bit means that it is difficult to do a titration of a weak acid against a weak base. Note: Because you almost never do titrations with this combination, there is no real point in giving the graph where they are added the other way round.
It isn't difficult to work out what it might look like if you are interested - take the beginning of the sodium hydroxide added to ethanoic acid curve, and the end of the ammonia added to hydrochloric acid one. The reason that it is difficult to do these titrations is discussed on the page about indicators. The way you normally carry out a titration involves adding the acid to the alkali. Here are reduced versions of the graphs described above so that you can see them all together. If the pH of an acid solution is plotted against the amount of base added during a titration, the shape of the graph is called a titration curve.
All acid titration curves follow the same basic shapes. In the beginning, the solution has a low pH and climbs as the strong base is added. The first curve shows a strong acid being titrated by a strong base. There is the initial slow rise in pH until the reaction nears the point where just enough base is added to neutralize all the initial acid.
This point is called the equivalence point. As the solution passes the equivalence point, the pH slows its increase where the solution approaches the pH of the titration solution. A weak acid only partially dissociates from its salt. The pH will rise normally at first, but as it reaches a zone where the solution seems to be buffered, the slope levels out. There are two main points to notice about this curve. The first is the half-equivalence point. The half-equivalence point is when just enough base is added for half of the acid to be converted to the conjugate base.
The second point is the higher equivalence point. When a weak acid is neutralized, the solution that remains is basic because of the acid's conjugate base remains in solution. These acids are called polyprotic acids.
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