Gas and Liquid Equilibrium Questions




Question-1. Compute the dissolved concentration of carbon monoxide (CO) that would be in equilibrium with a gas phase containing 0.1ppm CO by volume. Express the determined concentration in mg CO/L.

Solution Strategy:

  1. Write the chemical reaction for CO distribution between the aqueous and gas phases (see concept of Henry’s Law).
  2. Write the Henry’s Law constant (H) for the equation written in step-1.
  3. Find the value of H from the literature
  4. Knowing H and PCO(g), calculate CO(aq) in mol/L. For this step, make sure to convert the CO concentration from ppmv to either bar or atm units depending on the units of H.
  5. Convert the concentration of CO(aq) from mol/L to mg/L

Question-2. (1).Using the charge balance equation and the excel spreadsheet approach; determine the range of pH for a water in a cloud that has equilibrated with 1.0 ppm by volume SO2. The following is given:



(2). Calculate the pH of the same water using Visual MINTEQ.

Solution Strategy

To calculate the pH using the charge balance, proceed as in chapters 5 and 6 when we dealt with acids and bases.

  1. Identify the potential species that will be present in this system at equilibrium
  2. Write all relevant chemical reactions and corresponding equilibrium constants
  3. Write the mass balance equation for TOTSO3 and use the given in the problem statement to calculate the total concentration of SO3 in mol/L (proceed as in problem-1)
  4. Write the charge balance equation
  5. Solve for pH using the excel spreadsheet as requested


2. Using Visual Minteq.

  1. Open MINTEQ
  2. On the top bar, click on “GASES”, then “Specify F1”
  3. Select the second choice (Other gases).
  4. From the drop down window select SO2(g) and enter the concentration SO2 given in the problem statement, but in atm. units.
  5. Click on “ADD”.
  6. Go back to the main menu
  8. Output results and read pH

Question-3. A groundwater supply is initially at pH 8.0 and contains 2*10-3 eq/L alkalinity. If the alkalinity is all attributable to the carbonate system, determine whether the water is under-saturated or supersaturated with respect to atmospheric CO2

Solution Strategy

Write the formula for the determination of alkalinity

Alkalinity = [OH] + [HCO3] + 2[CO32-] – [H+]

  1. At pH 8, [HCO3] dominates all the other terms on the right side of the equation. Therefore, alkalinity can be equated with [HCO3] as follows:

ALK = [HCO3]

  1. Determine whether the water is under-saturated or supersaturated with respect to atmospheric CO2. To do so, assume ideal solute behavior to use concentrations and:

3(a). Write the chemical reaction of the dissociation of H2CO3 and the corresponding equilibrium constant (find value of K in HW-6). Next, derive the concentration of H2CO3 at equilibrium from the K-equation and the given pH

3(b) Next, calculate the concentration of H2CO3 when the system becomes in equilibrium with the atmosphere using Henry’s Law constant equation (i.e. H = PCO2g/[H2CO3]aq)

3(c). Compare the concentrations of H2CO3 obtained in parts 3(b) and 3(c) and determine the direction in which CO2 will go, i.e. dissolve in water as CO2(aq) or volatilize from water as CO2(g).

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