** EES6208 – PRINCIPLES OF WATER CHEMISTRY **

**GAS-LIQUID EQUILIBRIUM**

**HOMEWORK-07**

** 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: **

- Write the chemical reaction for CO distribution between the aqueous and gas phases (see concept of Henry’s Law).
- Write the Henry’s Law constant (
**H**) for the equation written in step-1. - Find the value of
**H**from the literature - Knowing
**H**and*P*, 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._{CO(g)} - 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 SO

_{2}. 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.

- Identify the potential species that will be present in this system at equilibrium
- Write all relevant chemical reactions and corresponding equilibrium constants
- 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)
- Write the charge balance equation
- Solve for pH using the excel spreadsheet as requested

**2. Using Visual Minteq. **

- Open MINTEQ
- On the top bar, click on “GASES”, then “Specify F1”
- Select the second choice (Other gases).
- From the drop down window select SO
_{2(g) }and enter the concentration SO2 given in the problem statement, but in atm. units. - Click on “ADD”.
- Go back to the main menu
- RUN MINTEQ.
- 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 CO

_{2}

**Solution Strategy**

Write the formula for the determination of alkalinity

**Alkalinity = [OH ^{–}] + [HCO_{3}^{–}] + 2[CO_{3}^{2-}] – [H^{+}]**

- At pH 8, [HCO
_{3}^{–}] dominates all the other terms on the right side of the equation. Therefore, alkalinity can be equated with [HCO_{3}^{–}] as follows:

** ALK = [HCO _{3}^{–}] **

- Determine whether the water is under-saturated or supersaturated with respect to atmospheric CO
_{2}. To do so, assume ideal solute behavior to use concentrations and:

**3(a).** Write the chemical reaction of the dissociation of H_{2}CO_{3} and the corresponding equilibrium constant (find value of K in HW-6). Next, derive the concentration of H_{2}CO_{3 }at equilibrium from the K-equation and the given pH

**3(b)** Next, calculate the concentration of H_{2}CO_{3 }when the system becomes in equilibrium with the atmosphere using Henry’s Law constant equation (i.e. **H **= P_{CO2g}/[H_{2}CO_{3}]_{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 CO_{2(aq) } or volatilize from water as CO_{2(g)}.