Question

In: Chemistry

A wastewater is simulated as a solution containing 10-2.7 M NaHCO3 and 10-3 M NH4Cl. a....

A wastewater is simulated as a solution containing 10-2.7 M NaHCO3 and 10-3 M NH4Cl.

a. List the dissolved species that you expect to be present in solution.

b. Prepare a log C-pH diagram for the system, and calculate the pH of the solution.

c. In order to remove nitrogen by volatilizing NH3 gas, 148 mg/L Ca(OH)2 (lime) will be added to raise the pH and convert NH4+ to the NH3(aq) form. Calculate the pH after lime addition. (Assume that no CaCO3 precipitates or ammonia loss takes place during lime addition.

d. If CaCO3 precipitation did occur during lime addition, would it raise, lower, or not change the pH of the solution? (Hint: Consider what species are removed from solution by this reaction, and what the response would be of the species that remain.)

e. Would removal of NH3(aq) by volatilization raise, lower, or not change the pH of the solution? Explain your reasoning briefly.

Draw the log C – pH diagram in excel, and solve parts b and c using proton conditions. Give reasons for parts d and e.

Solutions

Expert Solution

Solution:

Solution:

Wastewater comprises of 10-2.7 M NaHCO3 and 10-3 M NH4Cl

NaHCO3 + NH4Cl -------> NaCl + NH3 + CO2 + H2O

Log C-pH diagram:

10-2.7 M NaHCO3

[Na+] [HCO3-] = 10-2.7

NaHCO3 ------->Na+ + HCO3-

The possible reaction can be:

H2CO3 ------> H+ + HCO3-

   HCO3- -------> H+ + CO32-

H2O -------> H+ + OH-

   pH = [Na+] [HCO3-] / NaHCO3

pH = - log [OH-]

= - log [10-2.7] = 2.7

Equilibrium Constants can be expresed as:

K1 (of eq 1) = [H+] x [HCO3-]/ [H2CO3]

= 4.3 x 10-7

     K2 (of eq 2) = [H+] x [CO32-]/ [HCO3-]

= 5.6 x 10-11

10-3 M NH4Cl

[NH3+] [HCl-] = 10-3

HCl -----> H+ + Cl-

   NH4 -------> NH3 + H+

   NH3 --------> NH4+ + OH-

      pH = [NH3+] [HCl-] / NH4Cl

pH = log [10-3]

= -3

In a Log C-pH diagram, log C is plotted on the X-axis and Y-axis holds the pH value

Now, -pH = log [H+] and  log [OH-] = pH - pKw

CT = [NH3+] [NaCl-]

and the graph can be plotted according to the values provided above.

b) NaHCO3 + NH4Cl -------> NaCl + NH3 + CO2 + H2O

In NH4Cl , ammonia (NH3) is a weak base and the strong acid is HCl

In NaHCO3, NaOH acts as a strong base and H2CO3 is weak acid

  NH4Cl is a dominant species


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