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FE Review for Environmental Engineering. Problems, problems, problems Presented by L.R. Chevalier, Ph.D., P.E. Department of Civil and Environmental Engineering Southern Illinois University Carbondale. FE Review for Environmental Engineering. Chemical Foundations. I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
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1. FE Review for Environmental Engineering Problems, problems, problems Presented by L.R. Chevalier, Ph.D., P.E. Department of Civil and Environmental Engineering Southern Illinois University Carbondale

2. FE Review for Environmental Engineering Chemical Foundations

3. Calculate the molecular weight, equivalent weight, molarity and normality of the following: a. 200 mg/L HCl b. 150 mg/L H2SO4 c. 100 mg/L Ca(HCO3)2

4. Use periodic table to get molecular weight • Convert mg/L to mol/L • Determine n for each compound • Apply equations • EW = MW/n • N = Mn

5. “in an Acid/Base reaction, n is the # of hydrogen ions that a molecule transfers”

6. Convert 200 mg/L HCl to ppm

7. 200 mg/L = 200 ppm

8. Convert 300 ppm Mg 2+ to mg/L as CaCO3 • Convert 30 mg/L Mg2+ as CaCO3 to mg/L Note: MW Mg2+ is 24.31 g/mol

9. Determine the molecular weight of the species • Determine n • Equate EW=MW/n • Apply equation

10. Convert 300 ppm Mg 2+ to mg/L as CaCO3 300 ppm = 300 mg/L EW Mg2+ = 24.31/2 = 12.16 g/eq (300)(50/12.16) = 1233.55 mg/L as CaCO3

11. b) Convert 30 mg/L Mg2+ as CaCO3 to mg/L (30 mg/L as CaCO3)(12.16/50) = 7.3 mg/L

12. Balance the following chemical equations: CaCl2 + Na2CO3CaCO3 + NaCl C6H12O6 + O2 CO2 + H2O NO2+H2O HNO3 + NO

13. CaCl2 + Na2CO3CaCO3 + 2NaCl C6H12O6 + 6O2 6CO2+ 6H2O 3NO2+H2O 2HNO3+ NO

14. What is the pH if [H+] = 10-3? • pH = 3 • What is the pOH if [OH-] = 10-8? • pOH = 8 • What is the pH if [OH-] = 10-8? • pH = 14 - 8 = 6 • What is the [H+] if [OH-] = 10-5? • [H+]=105-14 = 10-9 mol/L

15. Derive a proof that in a neutral solution, the pH and the pOH are both equal to 7.

16. Evaluate the governing equation

17. Find the hydrogen ion concentration and hydroxide ion concentration in tomato juice having a pH of 4.1

18. Review how to convert • [H+]=10-pH • 10-4.1 mol/L = 7.94 x 10-5 mol/L • Review governing equation

19. ...... end of example

20. What percentage of total ammonia (i.e. NH3 + NH4+) is present as NH3 at a pH of 7? The pKa for NH4+ is 9.3.

21. The problem is asking: However, this expression has two unknowns. Therefore, we need a second equation. ?????

22. The problem is asking: Second Equation

23. Recall, pH=7 means [H] = 10-7

24. Therefore:

25. Remove nitrogen to prevent the stimulation of algae growth • Prevent excessive nitrate [NO3-] level in drinking water from causing a potentially lethal condition in babies known as methemoglobinemia Consider the problem of removing nitrogen from municipal wastewater

26. When organic matter decomposes, nitrogen is first released in the form of ammonia • NH3- low solubility in water (ammonia) • NH4+- highly soluble in water (ammonium ion) One way to remove is a process known as ammonia stripping

27. By driving the equilibrium toward the right, less soluble gas is formed and encouraged to leave the solution and enter air stream in a gas stripping tower. • This technique has been adapted for use in removing VOC’s (volatile organic chemicals) from groundwater. • How can the reaction be driven to the formation of ammonia (NH3)? • Need to decrease [H+] or increase the pH.

28. Highly Soluble Low Solubility Want to consider [NH3]/[NH4+] Should we decrease this or increase this?

29. Highly Soluble Low Solubility Increase it. How can we do this?

30. Highly Soluble Low Solubility Reduce [H+] Increase pH.

31. Let’s start here:

32. ----- end of example.

33. Summary Of Example Problem • Nitrogen, in the form of ammonia (NH3) is removed chemically from the water by raising the pH • This converts ammonium ion (NH4+) into ammonia • NH3 is then stripped from the water by passing large quantities of air through the water

34. A sample of water at pH 10 has 32.0 mg/L of carbonate and 56.0 mg/L of bicarbonate ion. Find the alkalinity as CaCO3.

35. Determine the MW of HCO3- and CO3-2 • Determine the EW of HCO3- and CO3-2 • Convert the concentrations of HCO3- , CO3-2, H+ and OH- to mg/L as CaCO3 • Add the concentrations in mg/L as CaCO3 of HCO3- , CO3-2, and OH-, and subtract H+

36. Now we need to convert to mg/L CaCO3

37. I will leave it up to you to check calculations for H+ and OH- ...... end of problem

38. The solubility product for the dissociation of Mg(OH)2 is 9 x 10-12. Determine the concentration of Mg2+ and OH- at equilibrium.

39. Write the equation for the reaction • Write the solubility product equation • Recognize from Eqn. 1 the relationship between the number of moles of Mg2+ and the number of moles of OH- resulting from the dissociation of Mg(OH)2, and how this relates to Eqn 2

40. 1. Write the equation for the reaction. 2. The solubility product equation is:

41. 3. If x is the amount of Mg2+ resulting from the dissociation is given as x, then the amount of OH- is equal to 2x. .....end of example

42. Magnesium is removed from an industrial waste stream by hydroxide precipitation at a pH = 10. Determine the solubility of Mg2+ in pure water at 25° C and pKsp of 10.74.

43. 1. Identify the two governing equations (Ksp and Kw) 2. Recognize that [OH-] = 10-14+pH 3. Substitute to derive an equation [Mg2+] = f(pH)