Category archives: Polyprotic acid problems

Diprotic and polyprotic acids contain multiple acidic protons that dissociate in distinct, sequential steps. As their name suggests, polyprotic acids contain more than one acidic proton. Two common examples are carbonic acid H 2 CO 3which has two acidic protons and is therefore a diprotic acid and phosphoric acid H 3 PO 4which has three acidic protons and is therefore a triprotic acid.

Diprotic and polyprotic acids show unique profiles in titration experiments, where a pH versus titrant volume curve clearly shows two equivalence points for the acid; this is because the two ionizing hydrogens do not dissociate from the acid at the same time. With any polyprotic acid, the first amd most strongly acidic proton dissociates completely before the second-most acidic proton even begins to dissociate.

Titration curve of carbonic acid : The titration curve of a polyprotic acid has multiple equivalence points, one for each proton.

Polyprotic Acid Example Chemistry Problem

A diprotic acid here symbolized by H 2 A can undergo one or two dissociations depending on the pH. Dissociation does not happen all at once; each dissociation step has its own K a value, designated K a1 and K a2 :. The first dissociation constant is necessarily greater than the second i. For example, sulfuric acid H 2 SO 4 can donate two protons in solution:. This first dissociation step of sulfuric acid will occur completely, which is why sulfuric acid is considered a strong acid; the second dissociation step is only weakly dissociating, however.

Take, for example the three dissociation steps of the common triprotic acid phosphoric acid:. For example, a generic diprotic acid will generate three species in solution: H 2 A, HA —and A 2-and the fractional concentration of HA —which is given by:. The following formula shows how to find this fractional concentration of HA —in which pH and the acid dissociation constants for each dissociation step are known:.

Fractional ion calculations for polyprotic acids : The above complex equations can determine the fractional concentration of various ions from polyprotic acids. Solve equilibrium problems using the appropriate approximations for weak and strong polyprotic acids. Polyprotic acids can lose more than one proton.

Polyprotic Acids

When determining equilibrium concentrations for different ions produced by polyprotic acids, equations can become complex to account for the various components. For a diprotic acid for instance, we can calculate the fractional dissociation alpha of the species HA — using the following complex equation:.

polyprotic acid problems

Equation for finding the fractional dissociation of HA- : The above concentration can be used if pH is known, as well as the two acid dissociation constants for each dissociation step; oftentimes, calculations can be simplified for polyprotic acids, however.Polyprotic acids can lose more than one proton. When determining equilibrium concentrations for different ions produced by polyprotic acids, equations can become complex to account for the various components. For a diprotic acid for instance, we can calculate the fractional dissociation alpha of the species HA — using the following complex equation:.

We can simplify the problem, depending on the polyprotic acid.


The following examples indicate the mathematics and simplifications for a few polyprotic acids under specific conditions. Because the first dissociation is so strong, we can assume that there is no measurable H 2 SO 4 in the solution, and the only equilibrium calculations that need be performed deal with the second dissociation step only.

At a pH equal to the pK a for a particular dissociation, the two forms of the dissociating species are present in equal concentrations, due to the following mathematical observation.

Take for instance the second dissociation step of phosphoric acid, which has a pK a2 of 7. As long as the pK a values of successive dissociations are separated by three or four units as they almost always arematters are simplified. When a weak diprotic acid such as carbonic acid, H 2 CO 3dissociates, most of the protons present come from the first dissociation step:. Boundless vets and curates high-quality, openly licensed content from around the Internet.

This particular resource used the following sources:. Skip to main content. Acids and Bases. Search for:. Calculating Equilibrium Concentrations of Polyprotic Acids.

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Learning Objective Solve equilibrium problems using the appropriate approximations for weak and strong polyprotic acids. Key Points Polyprotic acids contain multiple acidic protons that can sequentially dissociate from the compound with unique acid dissociation constants for each proton.

Due to the variety of possible ionic species in solution for each acid, precisely calculating the concentrations of different species at equilibrium can be very complicated.

Certain simplifications can make the calculations easier; these simplifications vary with the specific acid and the solution conditions. Show Sources Boundless vets and curates high-quality, openly licensed content from around the Internet.

Licenses and Attributions. CC licensed content, Shared previously.Weak Acid Weak Base pH calculation solved example. Weak Acids and Bases - Calculate the pH of a weak acid. Self-ionization of water - Autoionization of water - The ion product of water kw. Sometimes it is inconvenient to use the concentration units. If we are interested in viewing the progress of the reaction graphically, ph calculator. A large number of acids can give two or more protons on ionization dissociation and these are referred to as polyprotic acids.

For example, with sulfurous acid H 2 SO 3 we have the successive ionizations:. A polyprotic acid always dissociates in a stepwise manner, one proton at a time. Note that the acid dissociation constants are labelled k a1 and k a2. The numbers on the constants refer to the particular proton of the acid that is ionizing. Thus, k a1 always refers to the equilibrium involving removal of the first proton of a polyprotic acid. Note also that k a2 for sulfurous acid is much smaller than k a1.

The above observation is general: It is always easier to remove the first proton from a polyprotic acid than to remove the second and so on. The k a values become successively smaller as successive protons are removed. The acid dissociation constants for common polyprotic acids are given in Table I.

Table I.

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Depending on the pH of the solution, a polyprotic acid may exist predominantly as the undissociated acid or any one of its anionic forms. As an example let us calculate the fraction of phosphoric acid present as a function of the pH. Phosphoric acid is typical of most weak polyprotic acids in that its successive k a values are very different.

From the ionic equilibria shown above it is obvious that the relative acid strengths are:. This greatly simplifies the pH calculations for phosphoric acid solutions as is illustrated in the Example I. These are.Titrations are typically used for acid-base reactions and redox reactions. Here's an example problem determining the concentration of an analyte in an acid-base reaction:. A 25 ml solution of 0.

What was the concentration of the HCl? The equation would now be:. Different methods are used to determine the equivalence point of a titration. No matter which method is used, some error is introduced, so the concentration value is close to the true value, but not exact. For example, if a colored pH indicator is used, it might be difficult to detect the color change. Usually, the error here is to go past the equivalence point, giving a concentration value that is too high.

Another potential source of error when an acid-base indicator is used is if water used to prepare the solutions contains ions that would change the pH of the solution. For example, if hard tap water is used, the starting solution would be more alkaline than if distilled deionized water had been the solvent. If a graph or titration curve is used to find the endpoint, the equivalence point is a curve rather than a sharp point.

The endpoint is a sort of "best guess" based on the experimental data. The error can be minimized by using a calibrated pH meter to find the endpoint of an acid-base titration rather than a color change or extrapolation from a graph.

polyprotic acid problems

Share Flipboard Email. By Todd Helmenstine. Todd Helmenstine is a science writer and illustrator who has taught physics and math at the college level. He holds bachelor's degrees in both physics and mathematics. Updated January 24, Step 1: Determine [OH - ]. Step 2: Determine the number of moles of OH. Step 4: Determine the concentration of HCl. The concentration of the HCl is 0. The above steps can be reduced to one equation:.

For the example problem, the ratio is A diprotic acid dissociates in water in two stages:. Because of the successive dissociations, titration curves of diprotic acids have two equivalence points, as shown in Figure 1. The equations for the acid-base reactions occurring between a diprotic acid, H 2 X, and sodium hydroxide base, NaOH, are:.

Therefore, the volume of NaOH added at the second equivalence point is exactly twice that of the first equivalence point see Equations 3 and 5.

The primary purpose of this experiment is to identify an unknown diprotic acid by finding its molecular weight. A diprotic acid is titrated with NaOH solution of known concentration. Weighing the original sample of acid will tell you its mass in grams. Moles can be determined from the volume of NaOH titrant needed to reach the first equivalence point. Moles of unknown acid equal moles of NaOH at the first equivalence point see Equation 3.

Molecular weight determination is a common way of identifying an unknown substance in chemistry. You may use either the first or second equivalence point to calculate molecular weight. If the second equivalence point is more clearly defined on the titration curve, however, simply divide its NaOH volume by 2 to confirm the first equivalence point; or from Equation 5, use the ratio:.

Weigh out about 0. Record the mass to the nearest 0. Transfer the unknown acid to a mL beaker and dissolve in mL of distilled water. Place the beaker on a magnetic stirrer and add a stirring bar. If no magnetic stirrer is available, you need to stir with a stirring rod during the titration. Use a utility clamp to suspend a pH electrode on a ring stand as shown in Figure 2. Position the pH electrode in the HCl solution and adjust its position toward the outside of the beaker so that it is not struck by the stirring bar.

Use a utility clamp to attach the buret to the ring stand as shown in Figure 2. Fill the buret a little above the 0. Dispose of the waste solution in this step as directed by your teacher. You are now ready to begin the titration. This process goes faster if one person manipulates and reads the buret while another person operates the calculator and enters buret readings.

NaOH volume. As you move the cursor right or left, the volume X and pH Y are displayed below the graph. One of the two equivalence points is usually more clearly defined than the other; the two-drop increments near the equivalence points frequently result in larger increases in pH a steeper slope at one equivalence point than the other.

Polyprotic Acid Definition in Chemistry

Indicate the more clearly defined equivalence point first or second in Box 1 of the Data and Calculations table. Determine the volume of NaOH titrant used for the equivalence point you selected. To do so, examine the data to find the largest increase in pH values during the 2-drop additions of NaOH.

polyprotic acid problems

Find the NaOH volume just before this jump. Then find the NaOH volume after the largest pH jump.

polyprotic acid problems

Record these values in Box 2 of your data table. For the alternate equivalence point the one you did not use in the previous stepexamine the data points on your graph to find the largest increase in pH values during the 2 drop additions of NaOH. Find the NaOH volume just before and after this jump. Record these values in Box 10 of your data table. Dispose of the beaker and buret contents as directed by your teacher. Rinse the pH electrode with distilled water and return it to the storage solution.

Use your graph and data table to confirm the volumes you recorded in Box 2 of the Data and Calculations table volumes of NaOH titrant before and after the largest increase in pH values.Many acids contain two or more ionizable hydrogens. For any such multiple hydrogen acid, the first hydrogen is most easily removed, and the last hydrogen is removed with the greatest difficulty.

These acids are called polyprotic many protons acids. The multiple acid ionization constants for each acid measure the degree of dissociation of the successive hydrogens.

Table 1 gives ionization data for four series of polyprotic acids. The integer in parentheses after the name denotes which hydrogen is being ionized, where 1 is the first and most easily ionized hydrogen. Remember : The strongest acids dissociate most readily. Of the nine acids listed in Tablethe strongest is sulfuric 1with the highest acid ionization constant, and the weakest is phosphoric 3. Consequently, an aqueous solution of phosphoric acid contains all the following molecules and ions in various concentrations:.

Consulting the table of the dissociation constants K a 's for phosphoric acid shows that the first dissociation is much greater than the second, abouttimes greater. The and ions are present in very small concentrations. Previous Quiz Two Types of Bases. Next Quiz Polyprotic Acids. Removing book from your Reading List will also remove any bookmarked pages associated with this title.

Acids and Bases: Titration Example Problem

Are you sure you want to remove bookConfirmation and any corresponding bookmarks? My Preferences My Reading List. Polyprotic Acids. Adam Bede has been added to your Reading List!Polyprotic acids are acids that will donate two or more hydrogen ions during an acid—base reaction. Both diprotic and triprotic acids are polyprotic acids. HCN has only one hydrogen ion, it is a monoprotic acid and not a polyprotic acid.

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