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: slightly basic, since NH4+ and HCO3- both can act as weak acids, but HCO3- can also act as a weak base with a larger \(K_{\mathrm{b}}\) value. b. Sodium dihydrogen phosphate: acidic, as H2PO4- is a weak acid and Na+ does not affect acidity/basicity. c. Sodium hydrogen phosphate: slightly basic, since HPO4^2- is the conjugate base of a weak acid and Na+ does not affect acidity/basicity. d. Ammonium dihydrogen phosphate: acidic, because both NH4+ and H2PO4- are weak acids with similar \(K_{\mathrm{a}}\) values. e. Ammonium formate: basic, since NH4+ is a weak acid and HCOO- is the conjugate base of a weak acid, having a larger \(K_{\mathrm{a}}\) value than NH4+.

Step-by-step solution

Identifying the Anions and Cations

In this step, we will identify the anions and cations for each salt and look up the \(K_{\mathrm{a}}\) values in Appendix 5. a. ammonium bicarbonate: NH4+ (cation) and HCO3- (anion) b. sodium dihydrogen phosphate: Na+ (cation) and H2PO4- (anion) c. sodium hydrogen phosphate: Na+ (cation) and HPO4^2- (anion) d. ammonium dihydrogen phosphate: NH4+ (cation) and H2PO4- (anion) e. ammonium formate: NH4+ (cation) and HCOO- (anion)

Determining the Acid/Base Character of Each Ion

In this step, we will decide the acid/base character of each ion, using their \(K_{\mathrm{a}}\) or \(K_{\mathrm{b}}\) values as appropriate. Recall that \(K_{\mathrm{a}}\) and \(K_{\mathrm{b}}\) are related by \(K_{\mathrm{a}}\times K_{\mathrm{b}}=K_{\mathrm{w}}=10^{-14}\). (a) NH4+ is a weak acid with \(K_{\mathrm{a}}\approx5.6\times10^{-10}\). HCO3- is an amphoteric ion (it can act as an acid or a base); it has the acidic dissociation constant \(K_{\mathrm{a1}}\approx4.8\times10^{-11}\) and its conjugate base, CO3^2-, has \(K_{\mathrm{b}}\approx 2.1\times10^{-4}\). (b) Na+ is a neutral ion and does not affect acidity and basicity directly. H2PO4- is a weak acid with \(K_{\mathrm{a}}\approx6.2\times10^{-8}\). (c) Na+ is a neutral ion. HPO4^2- is the conjugate base of a weak acid (H2PO4-) with \(K_{\mathrm{b}}\approx 2.4\times10^{-13}\). (d) NH4+ is a weak acid with \(K_{\mathrm{a}}\approx5.6\times10^{-10}\). H2PO4- is a weak acid with \(K_{\mathrm{a}}\approx6.2\times10^{-8}\). (e) NH4+ is a weak acid with \(K_{\mathrm{a}}\approx5.6\times10^{-10}\). HCOO- is the conjugate base of a weak acid, formic acid, and has \(K_{\mathrm{a}}\approx1.77\times10^{-4}\).

Comparing the Acid/Base Characters of the Salts

In this step, we will compare the acidity and basicity of the anions and cations, and classify the resulting salt solutions as acidic, basic, or neutral. a. NH4+ and HCO3- both can act as weak acids, but HCO3- can also act as a weak base. Given that the \(K_{\mathrm{a}}\) value of NH4+ is larger than the \(K_{\mathrm{a1}}\) value of HCO3- and the \(K_{\mathrm{b}}\) value of HCO3- is much larger than the \(K_{\mathrm{a}}\) value of NH4+, the overall solution may be slightly basic. b. Na+ does not affect acidity/basicity. H2PO4- is a weak acid and will contribute protons to the solution, making it acidic. c. Na+ does not affect acidity/basicity. HPO4^2- is the conjugate base of a weak acid and will create an equilibrium for accepting protons, making the overall solution slightly basic. d. Both NH4+ and H2PO4- are weak acids with \(K_{\mathrm{a}}\) values close to each other, so their acidic characters will add up, making the overall solution acidic. e. NH4+ is a weak acid and HCOO- is the conjugate base of a weak acid, having a \(K_{\mathrm{a}}\) much larger than that of NH4+. Therefore, the basic character of HCOO- will dominate, and the overall solution will be basic. So, the final answer is: a. basic b. acidic c. basic d. acidic e. basic

Key concepts

Salt Solutions

In acid-base chemistry, salt solutions are aqueous solutions containing dissolved salts. When salts dissolve in water, they split into their constituent anions and cations.
These ions can affect the acidity or basicity of the solution. For example, ammonium bicarbonate \(\rm (NH_4HCO_3)\) consists of the cation \(\rm NH_4^+\) and the anion \(\rm HCO_3^-\). Upon dissolving in water, these ions determine whether the solution is acidic, basic, or neutral based on their individual properties. Different ions have different effects:

• Neutral ions like \(\rm Na^+\) do not affect pH.
• Weak acids like \(\rm NH_4^+\) release protons, making the solution acidic.
• Amphoteric ions like \(\rm HCO_3^-\) can act as either acids or bases, depending on the environment.
• The balance of these effects determines the nature of the solution.

Understanding how each ion behaves helps in predicting the pH nature of salt solutions.

Equilibrium

Equilibrium in chemistry refers to a state where the forward and reverse reactions occur at the same rate. For salt solutions, particularly in acid-base chemistry, this involves the dissociation and interaction of ions in water.For example, if we take sodium dihydrogen phosphate (\( \rm NaH_2PO_4 \)), it dissociates in water to form \(\rm Na^+\) and \(\rm H_2PO_4^-\). The \(\rm H_2PO_4^-\) ion can lose a proton, contributing to the acidity of the solution, as it reaches equilibrium in a series of dissociation steps.The equilibrium position is crucial in determining the final pH, since it reflects the proportions of ions released in the solution:

• If more \(\rm H^+\) ions are released, the solution is acidic.
• If more \(\rm OH^-\) ions are generated, the solution is basic.
• When neither predominates significantly, the solution remains closer to neutral.

This dynamic balance is key to understanding the behavior of salt solutions.

Ka and Kb Values

In acid-base chemistry, \(K_a\) and \(K_b\) values are constants that measure the strength of acids and bases, respectively. The \(K_a\) value is the acid dissociation constant, while \(K_b\) is the base dissociation constant.These values help predict how much an acid or base will dissociate in water:

• A larger \(K_a\) value indicates a stronger acid, meaning it dissociates more in solution.
• A larger \(K_b\) value indicates a stronger base with a similar implication for dissociation.
• The product of \(K_a\) and \(K_b\) of a conjugate acid-base pair is always \(K_w\), \(10^{-14}\).

By comparing \(K_a\) and \(K_b\) values, we can predict the pH of solutions. For instance, if \(K_a\) for ammonium ion \(\rm (NH_4^+)\) is higher than the \(K_b\) for bicarbonate \(\rm (HCO_3^-)\), the solution tends towards acidity.These constants act as a guide to understand the acidic or basic potential of components within a salt solution.

Amphoteric Ions

Amphoteric ions are a unique group of ions that can act as both acids and bases. This dual nature allows them to either donate a proton or accept a proton, depending on the prevailing conditions in the solution. A classic example is the bicarbonate ion \(\rm (HCO_3^-)\):

• As an acid, \(\rm HCO_3^-\) can donate a proton to become \(\rm CO_3^{2-}\).
• As a base, it can accept a proton to form \(\rm H_2CO_3\).

Because of this amphoteric behavior, the outcome is dictated by the balance between the tendencies of the ions to gain or lose protons, contributing to the complexity of predicting the final pH.This behavior is important not just theoretically but also practically, as it directly affects buffering capacity in biological systems and industrial processes.Understanding amphoteric ions is crucial for accurately predicting the behavior of many salt solutions in acid-base reactions.