Question

The \(\mathrm{pH}\) of a particular raindrop is 5.6. (a) Assuming the major species in the raindrop are \(\mathrm{H}_{2} \mathrm{CO}_{3}(a q), \mathrm{HCO}_{3}^{-}(a q),\) and \(\mathrm{CO}_{3}^{2-}(a q),\) calculate the concentrations of these species in the raindrop, assuming the total carbonate concentration is \(1.0 \times 10^{-5} M .\) The appropriate \(K_{a}\) values are given in Table 16.3. (b) What experiments could you do to test the hypothesis that the rain also contains sulfur-containing species that contribute to its \(\mathrm{pH}\) ? Assume you have a large sample of rain to test.

Short answer

The concentrations of the species in the raindrop can be calculated by setting up and simultaneously solving mass balance, charge balance, and acid dissociation equations. Using the given pH of 5.6 and total carbonate concentration, we find the concentrations of [HCO3-] and [CO3^2-]. To test the hypothesis that the raindrop also contains sulfur-containing species, experiments such as filtration tests, chemical tests to detect sulfurous compounds, and the addition of a sulfur scavenger can be performed. These tests will help determine if a change in pH occurs after treatment, indicating the presence of sulfur-containing species.

Step-by-step solution

Write the mass balance equation

The total carbonate concentration in the raindrop is the sum of the concentrations of each form of carbonate. That is: Total Carbonate = [H2CO3] + [HCO3-] + [CO3^2-] = \(1.0 \times 10^{-3} M\)

Write the charge balance equation

The total positive charge should be equal to the total negative charge in the raindrop. We also have to consider the main source of hydrogen ion (\(H^{+}\)) and hydroxide ion (\(OH^{-}\)) concentrations when writing this equation. So, the charge balance equation can be written as: \[ [H^+] = [HCO_3^-] + 2[CO_3^{2-}] + [OH^-] \]

Write the acid dissociation equations

We are given two equilibrium constants for the dissociation of carbonic acid: \(K_{a1} = \frac{[H^+][HCO_3^-]}{[H_2CO_3]}\) \(K_{a2} = \frac{[H^+][CO_3^{2-}]}{[HCO_3^-]}\)

Use the pH to determine the hydrogen ion concentration

The pH of the raindrop is 5.6, so we can determine the hydrogen ion concentration as follows: \[ [H^+] = 10^{-pH} = 10^{-5.6} \]

Calculate the hydroxide ion concentration

We can find the hydroxide ion concentration from the ion product of water: \[ K_w = [H^+][OH^-] \] Where \(K_w = 10^{-14}\), so \[ [OH^-] = \frac{10^{-14}}{[H^+]} \]

Solve the equations

With all the needed equations, we can now solve the system of equations. We can substitute equations from Step 1, 3, and 4 into the charge balance equation (Step 2). We can then solve for [HCO3-] and [CO3^2-]. After determining the concentration of these species, we can find [H2CO3] using the mass balance equation. Part (b):

Suggested experiments to test the hypothesis

The hypothesis is that the raindrop contains sulfur-containing species that contribute to its pH. We can perform the following experiments: 1. Perform a filtration test: A simple filtration test may help in removing any solid-form sulfur compounds like sulfates. Measure the pH after filtration and compare it with the initial pH. 2. Conduct chemical tests to detect the presence of sulfurous compounds: Colorimetric, ion-selective electrode, or chromatography methods can detect the presence of sulfur-containing species, such as sulfates or sulfites, in the raindrop sample. 3. Add a sulfur scavenger: Adding a chemical that reacts with sulfur-containing species and removes them can help determine if the pH changes after treating the sample with the scavenger. If the pH changes, this indicates that sulfur-containing species contribute to the initial pH. These experiments can help in verifying the presence of sulfur-containing species in raindrops and determining their contribution to the pH values.

Key concepts

Carbonate Species

Rainwater naturally absorbs carbon dioxide ( CO_2 ) from the atmosphere, forming carbonic acid ( H_2CO_3 ). This acid partially dissociates into bicarbonate ( HCO_3^- ) and carbonate ions ( CO_3^{2-} ). Together, these species exist as a dynamic balance in rainwater, making them significant contributors to the water's pH.
To understand this balance, we use concentration changes of each species that are linked by mass and charge balance equations. The total carbonate concentration represents the sum of these species' concentrations. The interplay of these species determines the rainwater acidity and hence its pH.

Acid Dissociation Constant

The acid dissociation constant ( K_a ) is crucial in understanding how strong an acid is, by showing how completely it dissociates in water.
For carbonic acid in rain, we have two dissociation steps with their specific constants: K_{a1} and K_{a2} . These constants help us determine the concentrations of carbonate species by quantifying the conversion of H_2CO_3 into HCO_3^- , and then into CO_3^{2-} .

• K_{a1} applies to the release of the first hydrogen ion from H_2CO_3 , forming HCO_3^- .
• K_{a2} signifies the release of the second hydrogen ion from HCO_3^- , forming CO_3^{2-} .

Understanding these values allows us to predict the behavior of carbonate species and, subsequently, the pH of rainwater.

Hydrogen Ion Concentration

The hydrogen ion concentration ( [H^+] ) directly influences pH, with the relationship defined by the formula [H^+] = 10^{-pH} . For rain with a pH of 5.6, the corresponding hydrogen ion concentration is very critical in the calculations of the carbonate species' balance.
In raindrops, the concentration of hydrogen ions balances against HCO_3^- and CO_3^{2-} to maintain overall neutrality. Calculating [H^+] allows us to write charge balance equations reflecting real-world chemical equilibria.
This understanding is essential for quantifying other ions in the solution and predicting shifts in pH based on potential environmental changes.

Sulfur-Containing Species in Rain

Sulfur-containing species are potential contributors to the acidity of rainwater, mainly in the form of sulfates ( SO_4^{2-} ) and sulfites ( SO_3^{2-} ). They enter raindrops through industrial pollutants and natural sources like volcanoes. These species can lower the rain's pH by increasing the concentration of hydrogen ions when they dissolve and form strong acids, like sulfuric acid ( H_2SO_4 ).
To detect these influences, experiments such as **filtration tests** or **chemical analysis** can be performed. These tests can help in identifying the presence and impact of sulfur compounds in rain samples. Using such experiments can measure differences in pH before and after sulfur species are neutralized, further highlighting their role in rainwater's chemical dynamics.