Question

The 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 \(1.0 \times 10^{-5} \mathrm{M}\) . The appropriate \(K_{a}\) values are given in Table \(16.3 .\) (b) What experiments could you do to test the hypothesis the rain also contains sulfur-containing species that contribute to its pH? Assume you have a large sample of rain to test.

Short answer

To determine the concentrations of the major species in the raindrop, we first calculated the hydrogen ion concentration using the given pH. We then used the \(K_{a}\) values and equilibrium expressions to form a set of equations and solved for the unknown concentrations. The proposed experiments to detect sulfur-containing species in the rain sample include gravimetric analysis for sulfate ions, colorimetric method for sulfite ions, and acid-base titration for dissolved sulfur dioxide.

Step-by-step solution

Calculate hydrogen ion concentration using pH

To find the concentration of \(\mathrm{H}^{+}\) ions, we will use the formula for pH: \(pH = -\log [\mathrm{H}^{+}] \) Given, pH = 5.6. Now, we will calculate the hydrogen ion concentration: \[ [\mathrm{H}^{+}] = 10^{-pH} = 10^{-5.6} \]

Define equilibrium expressions involving carbonate species

Using the given \(K_{a}\) values and the equilibrium expressions for the dissolution of carbonic acid, we have: \( K_{a1} = \frac{[\mathrm{H}^{+}][\mathrm{HCO}_{3}^{-}]}{[\mathrm{H}_{2}\mathrm{CO}_{3}]} \) \( K_{a2} = \frac{[\mathrm{H}^{+}][\mathrm{CO}_{3}^{2-}]}{[\mathrm{HCO}_{3}^{-}]} \) Also, we are given the total carbonate concentration, \[ [\mathrm{H}_{2}\mathrm{CO}_{3}] + [\mathrm{HCO}_{3}^{-}] + [\mathrm{CO}_{3}^{2-}] = 1.0 \times10^{-5} M \]

Solve the equations for carbonate species concentrations

Now, we have three unknowns, and three equations. To solve for concentrations of \(\mathrm{H}_{2}\mathrm{CO}_{3}(a q), \mathrm{HCO}_{3}^{-}(a q),\) and \(\mathrm{CO}_{3}^{2-}(a q),\) we need to eliminate one variable. Let's substitute \([\mathrm{HCO}_{3}^{-}]\) from the first equilibrium expression into the second one: \( [\mathrm{HCO}_{3}^{-}] = \frac{[\mathrm{H}_{2}\mathrm{CO}_{3}] \times K_{a1}}{[\mathrm{H}^{+}]} \) Substituting into the second expression, we get: \( K_{a2} = \frac{[\mathrm{H}^{+}] [\mathrm{CO}_{3}^{2-}] }{ \frac{[\mathrm{H}_{2}\mathrm{CO}_{3}] \times K_{a1}}{[\mathrm{H}^{+}]}} \) \( K_{a2} [\mathrm{H}_{2}\mathrm{CO}_{3}] \times K_{a1} = [\mathrm{CO}_{3}^{2-}][\mathrm{H}^{+}]^{2} \) Now, we can substitute the terms back into the total carbonate concentration equation and solve for the concentrations of \(\mathrm{H}_{2}\mathrm{CO}_{3}(a q), \mathrm{HCO}_{3}^{-}(a q),\) and \(\mathrm{CO}_{3}^{2-}(a q).\) Given, \(K_{a1} = 4.45\times 10^{-7}\) and \(K_{a2} = 4.69\times 10^{-11}\). After substitution and simplification, we can find the concentration values of major species in the raindrop. #b. Proposed experiments to detect sulfur-containing species#

Testing for sulfate ions using gravimetry

One way to test for the presence of sulfur-containing species like sulfate ions in the rain sample is to perform gravimetric analysis. In this method, we can precipitate sulfate ions as barium sulfate by adding barium chloride solution. It forms an insoluble precipitate that can be filtered, dried, and weighed accurately. The mass of the precipitate can then be used to determine the sulfate concentration in the collected rain.

Testing for sulfite ions using colorimetric method

Sulfite ions in rainwater can be detected using a colorimetric method. In this method, the reaction of sulfite with an acidic solution of potassium iodate and starch will produce a blue-black color due to the formation of triiodide ions. The intensity of the color can be related to the concentration of sulfite ions using a calibration curve.

Testing for dissolved sulfur dioxide gas using acid-base titration

Another sulfur-containing species in rainwater is \(\mathrm{SO}_{2}\) gas. To test for \(\mathrm{SO}_{2}\), we can first bubble the collected rainwater sample through a solution of hydrogen peroxide, which will convert dissolved \(\mathrm{SO}_{2}\) to \(\mathrm{HSO}_{3}^{-}\). The resulting solution can then be titrated with a standard NaOH solution. The amount of NaOH used in the titration will indicate the concentration of dissolved sulfur dioxide in the rainwater sample.

Key concepts

Carbonate Equilibrium

In a raindrop, the carbonate equilibrium involves three main species: carbonic acid (\(\mathrm{H}_{2}\mathrm{CO}_{3}\)), bicarbonate (\(\mathrm{HCO}_{3}^{-}\)), and carbonate (\(\mathrm{CO}_{3}^{2-}\)). Understanding how these species interact is essential in calculating their respective concentrations.

The carbonate equilibrium is governed by the acid dissociation constants, given as \(K_{a1}\) and \(K_{a2}\). These constants are used in the equilibrium expressions:

• \(K_{a1} = \frac{[\mathrm{H}^{+}][\mathrm{HCO}_{3}^{-}]}{[\mathrm{H}_{2}\mathrm{CO}_{3}]}\)
• \(K_{a2} = \frac{[\mathrm{H}^{+}][\mathrm{CO}_{3}^{2-}]}{[\mathrm{HCO}_{3}^{-}]}\)

Using these equations, scientists can calculate the concentrations of each carbonate species when the total carbonate concentration is known, like the given \(1.0 \times 10^{-5} \mathrm{M}\).

By substituting and rearranging the equations, it is possible to solve for the concentrations of \(\mathrm{H}_{2}\mathrm{CO}_{3}\), \(\mathrm{HCO}_{3}^{-}\), and \(\mathrm{CO}_{3}^{2-}\) in the raindrop, ensuring we understand how equilibrium shifts affect the system parameters.

Acid-Base Equilibrium

Acid-base equilibrium refers to the balance of hydrogen ions (\(\mathrm{H}^{+}\)) in solution, which dictates the pH of a substance. In our raindrop example, with a pH of 5.6, it is slightly acidic.

To calculate the concentration of \(\mathrm{H}^{+}\) ions, you can use the formula:

• \(\mathrm{pH} = -\log [\mathrm{H}^{+}]\)
• \([\mathrm{H}^{+}] = 10^{-\mathrm{pH}}\)

Here, substituting 5.6 for pH, you find \([\mathrm{H}^{+}] = 10^{-5.6}\).

Acid-base equilibrium in the carbonate system involves keeping track of the dissociations that occur as \(\mathrm{H}_{2}\mathrm{CO}_{3}\) and its products, \(\mathrm{HCO}_{3}^{-}\) and \(\mathrm{CO}_{3}^{2-}\), release and uptake \(\mathrm{H}^{+}\). These changes impact the pH, and affect the equilibrium of the solution.

Sulfur-Containing Species Detection

To determine if sulfur-containing species contribute to the rain's acidity, several detection methods can be used:

- **Gravimetric Analysis**: Experimenting with sulfate ions involves precipitating them as barium sulfate. This insoluble precipitate allows the measurement of sulfate concentration after filtration, drying, and weighing.
- **Colorimetric Method**: Sulfite ions can be detected by their reaction with potassium iodate in an acidic solution, forming a blue-black color. The color intensity reflects the sulfite concentration when compared visually or with a spectrophotometer.
- **Titration for \(\mathrm{SO}_2\)**: Dissolved sulfur dioxide can be oxidized by hydrogen peroxide into bisulfite. This new compound is then titrated with NaOH, revealing its concentration through the volume of NaOH required.

These methods serve to confirm the presence of sulfur-related compounds, which can further clarify their impact on overall pH levels in rain.