Question

Will 0.10 M solutions of the following salts be acidic, basic, or neutral? See Appendix 5 for \(K_{\mathrm{a}}\) values. a. ammonium bicarbonate b. sodium dihydrogen phosphate c. sodium hydrogen phosphate d. ammonium dihydrogen phosphate e. ammonium formate

Short answer

a. Ammonium bicarbonate (\(NH_4HCO_3\)): Slightly basic b. Sodium dihydrogen phosphate (\(NaH_2PO_4\)): Basic c. Sodium hydrogen phosphate (\(Na_2HPO_4\)): Basic d. Ammonium dihydrogen phosphate (\(NH_4H_2PO_4\)): Slightly acidic e. Ammonium formate (\(NH_4HCO_2\)): Slightly acidic

Step-by-step solution

a. Ammonium bicarbonate

First, we write the salt's formula: \(NH_4HCO_3\). The cation is \(NH_4^+\) (ammonium), which comes from the weak base \(NH_3\) (ammonia) and the anion is \(HCO_3^-\) (bicarbonate) which comes from \(H_2CO_3\) (carbonic acid), a weak acid. Since the salt comes from both a weak base and weak acid, we must compare Kᵃ values to determine whether it will be acidic, basic, or neutral. For ammonia, the Kb =\(1.8 \times 10^{-5}\). To find the Ka value, we use the relationship \(K_w = K_a \times K_b\), where \(K_w = 1.0 \times 10^{-14}\). \(K_a = \frac{K_w}{K_b} = \frac{1.0 \times 10^{-14}}{1.8 \times 10^{-5}} = 5.6 \times 10^{-10}\). For carbonic acid, the Kᵃ1 = \(4.3 \times 10^{-7}\). Since \(K_{a1} > K_a\), the salt will be slightly basic.

b. Sodium dihydrogen phosphate

First, we write the salt's formula: \(NaH_2PO_4\). The cation is \(Na^+\) (sodium), which comes from a strong base (\(NaOH\)) and the anion is \(H_2PO_4^-\) (dihydrogen phosphate) which comes from a weak acid (\(H_3PO_4\), phosphoric acid). Since we have a weak acid and a strong base, the solution will be basic.

c. Sodium hydrogen phosphate

First, we write the salt's formula: \(Na_2HPO_4\). The cation is \(Na^+\) (sodium), which comes from a strong base (\(NaOH\)) and the anion is \(HPO_4^{2-}\) (hydrogen phosphate) which comes from a weak acid (\(H_2PO_4^-\)). Since we again have a strong base and a weak acid, this solution will also be basic.

d. Ammonium dihydrogen phosphate

First, we write the salt's formula: \(NH_4H_2PO_4\). The cation is \(NH_4^+\) (ammonium), which comes from the weak base \(NH_3\) (ammonia) and the anion is \(H_2PO_4^-\) (dihydrogen phosphate) which comes from \(H_3PO_4\) (phosphoric acid), a weak acid. Since the salt comes from both a weak base and weak acid, we must compare Kᵃ values to determine whether it will be acidic, basic, or neutral. We already found the Kᵃ for ammonia (\(5.6 \times 10^{-10}\)). For phosphoric acid, the Kᵃ1 = \(7.2 \times 10^{-3}\). Since \(K_{a1} > K_a\), this salt will be slightly acidic.

e. Ammonium formate

First, we write the salt's formula: \(NH_4HCO_2\). The cation is \(NH_4^+\) (ammonium), which comes from the weak base \(NH_3\) (ammonia) and the anion is \(HCO_2^-\) (formate) which comes from \(HCO_2H\) (formic acid), a weak acid. Since the salt comes from both a weak base and weak acid, we must compare Kᵃ values to determine whether it will be acidic, basic, or neutral. We already found the Kᵃ for ammonia (\(5.6 \times 10^{-10}\)). For formic acid, the Kᵃ = \(1.8 \times 10^{-4}\). Since \(K_{a} > K_a\), this salt will be slightly acidic.

Key concepts

Acidic Solutions

An acidic solution is one where the concentration of hydrogen ions (\(H^+\)) is higher than the concentration of hydroxide ions (\(OH^-\)). This type of solution has a pH less than 7. Salts can form acidic solutions if they are composed of a strong acid and a weak base. This happens because the cation from the weak base hydrolyzes, producing hydrogen ions. For instance, ammonium formate (\(NH_4HCO_2\)) forms an acidic solution. This is because ammonium (\(NH_4^+\)) comes from the weak base ammonia. When dissolved, ammonium can release \(H^+\) ions, making the solution slightly acidic. Comparing \(K_a\) values of components, if \(K_a\) of the acid part is higher, the solution tends to be acidic.

Basic Solutions

Basic solutions have a higher concentration of hydroxide ions (\(OH^-\)) than hydrogen ions (\(H^+\)), resulting in a pH greater than 7. Salts that lead to basic solutions usually consist of a strong base and a weak acid. Take sodium dihydrogen phosphate (\(NaH_2PO_4\)), for example. The sodium ion (\(Na^+\)) comes from a strong base, sodium hydroxide (\(NaOH\)). The dihydrogen phosphate ion (\(H_2PO_4^-\)) comes from phosphoric acid, a weaker acid. In this pairing, the weak acid cannot fully donate hydrogen ions to shift the balance towards neutrality, making the solution slightly basic. Additionally, base strength can be affirmed by examining \(K_b\) values, where significant strength indicates a basic solution.

Neutral Solutions

Neutral solutions occur when a salt is composed of both a strong acid and a strong base. Such solutions have a pH of exactly 7, indicating an equal concentration of \(H^+\) and \(OH^-\) ions. However, in our specific examples from the exercise, the focus was on salts resulting in either acidic or basic outcomes. In this setting, neutrality is less affected by hydrolysis processes that shift pH to either side. Generally, the formation of neutral solutions usually involves no significant hydrolysis, as the ions do not affect the pH, resulting in the neutrality property.

Ka Values

The \(K_a\) value represents the acid dissociation constant, a quantitative measure of the strength of an acid in solution. It is crucial when evaluating whether a solution will be acidic or basic. A larger \(K_a\) value indicates a stronger acid that dissociates more into \(H^+\) ions. For example, in the case of ammonium bicarbonate, the comparison revealed that the \(K_a\) for carbonic acid (\(4.3 \times 10^{-7}\)) was larger than that calculated for ammonia, indicating that the salt will be basic. This application shows how comparing \(K_a\) values can accurately predict the acidic or basic nature of a solution formed by salt.

Kb Values

\(K_b\) values indicate the base dissociation constant and are similar to \(K_a\) values but for bases. They inform us how well a base dissociates into \(OH^-\) ions in solution. A greater \(K_b\) signals a stronger base. When evaluating salts from weak acids and bases, such as ammonium bicarbonate, the interplay between \(K_a\) and \(K_b\) is essential in determining whether the solution is acidic or basic. The conversion between these constants, via the relationship \(K_w = K_a \times K_b\), where \(K_w = 1.0 \times 10^{-14}\), allows for these evaluations. Calculating \(K_b\) helps predict whether the solution leans more towards being basic, which is critical in understanding salt hydrolysis.