Calculate the concentration of SO2(ppm) that must be reached in polluted air (in ppm) if the dissolved gas is to produce a pH of 4.1 in raindrops without any oxidation of the gas.
Hint: Sulfurous acid equilibria
how would i do this.
what equations would i use.?
SO2 forms H2SO3 in water:
Step 1
H2SO3(aq) <=> H^+(aq) + HSO3^-(aq)
K1 = [H+][HSO3^-]/[H2SO3] = 1.71x10^-2
Step 2
HSO3-(aq) <=> H^+(aq) + SO3^-2(aq)
K2 = [H+][SO3^-2]/[HSO3^-] = 6.0x10^-8
The above sequence shows that Step 1 is the primary source of hydrogen ions and that we can ignore Step 2.
If the pH = 4.1, [H+]=7.94x10^-5
Substituting into the K1 expression,
[7.94x10^-5][7.94x10^-5]/[C-7.94x10^-5] = 1.71x10^-2
Solve for the molar concentration of H2SO3, C. Multiply the value of C by the molar mass of SO2 to get grams of SO2 per liter. Convert the last value to milligrams/liter which is equivalent to parts per million (ppm).
To calculate the concentration of SO2 (ppm) in polluted air, we need to use the equilibria of sulfurous acid. Sulfur dioxide (SO2) dissolves in water to form sulfurous acid (H2SO3), which dissociates to release hydrogen ions (H+) and bisulfite ions (HSO3-). The bisulfite ions further dissociate to form hydrogen ions (H+) and sulfite ions (SO3^2-). The equilibrium equations involved are:
1. SO2 + H2O ⇌ H2SO3
2. H2SO3 ⇌ H+ + HSO3-
3. HSO3- ⇌ H+ + SO3^2-
Given that the pH of the raindrops is 4.1, we can assume that the concentration of hydrogen ions (H+) is 10^(-4.1) M.
To calculate the concentration of SO2 (ppm), we need to follow these steps:
Step 1: Write the equilibrium expression for the dissociation of sulfurous acid (equation 2). Since H2SO3 is a weak acid, we can assume that the concentration of the undissociated species is equal to the initial concentration.
[H+] [HSO3-] / [H2SO3] = Ka
Step 2: Write the equilibrium expression for the dissociation of bisulfite ions (equation 3). Again, we can assume that the concentration of the undissociated species is equal to the initial concentration.
[H+] [SO3^2-] / [HSO3-] = Kb
Step 3: Use the given equilibrium constants Ka and Kb to solve for the concentrations of H2SO3 and HSO3-. Since we assume the initial concentration of H2SO3 to be equal to the concentration of SO2 in the polluted air (in ppm), we can substitute the concentration of H2SO3 obtained from equation 1 into equations 2 and 3 to solve for the concentrations of HSO3- and SO3^2-.
Step 4: Calculate the concentration of SO2 (ppm) by converting the concentration of H2SO3 (M) into ppm units.
Note: The calculation requires the values of Ka and Kb, which depend on temperature. You can refer to a chemistry reference book or an online database to obtain these values for a specific temperature.
It's also important to note that this calculation assumes no oxidation of the gas. If the gas undergoes oxidation, the equilibrium will shift, resulting in different concentrations of the species involved.