Question

The pH of clean rain was calculated to be 5.68. Acid rain, defined here as rain with pH less than 5.68, can be formed when sulfur dioxide (SO₂) is discharged into the atmosphere. Assume that the atmosphere contains 20 ppb by volume of SO₂ (10^-7.7 atm). Develop an equation that will allow determination of the pH of rainwater in equilibrium with CO₂ in air (at 10^-3.5 atm) and SO₂. Use a spreadsheet to calculate this pH. Comment on the relative impacts of SO₂ and CO₂ on the pH of rain. The following should be used.

H₂SO₃(aq) = SO₂ (g) + H₂O (K_H,SO2 = 0.80 atm-L/mol)

H₂SO₃ = HSO₃^- + H⁺ (pK_A1 = 1.77)

HSO₃^- = SO₃^2- + H₂O (pK_A2 = 7.21)

Answer 1

Given equation:

H2SO3(aq) SO2(g) + H2O

Do the similar steps for H2CO3 when rainwater is in equilibrium with CO2,

Also, 20 ppb(parts per billion) is equal to 0.02 mg/l.

Given info, pH(Clean water) = 5.68

KH = 0.80 atm L/ mol

Now, P_SO2 = 0.02× (10-7.7) atm

Also, [H2SO3] = KH × P_SO2 = 0.80× 0.02× (10-7.7) M

For equation, H2SO3 H+ + HSO3-. pka1= 1.77

We assume that [H+] = [HSO3-]

So, pka1= 1.77= [H+] [HSO3-]/ [[H2SO3]

1.77 = [H+]²/ [H2SO3]

Given, pH= 5.68

So [H+] = 10^-5.68

Also, pka2= 7.21= [SO32-]/ [H+]

Calculating above, we will get [SO32-]