CE 347 – Spring 2021 Homework #2 Due February 19, 11:59 PM Be sure to show all work. 1) (5 points) Write the equilibrium constant equations for the following reactions (hint: make sure each chemical equation is balanced!): a) CO2 + H2O ↔ H+ + HCO3b) Al2(SO4)3 ↔ Al3+ + SO422) (5 points) Acid mine drainage occurs when pyrite (FeS2; also known as Fool’s Gold) reacts with air and water. Balance the following equation describing this reaction and find the theoretical oxygen demand (in mg/L of O2) needed to completely oxidize 25 mg/L of pyrite. 𝐹𝑒𝑆2 + 𝑂2 + 𝐻2 𝑂 → 𝐹𝑒𝑆𝑂4 + 𝐻2 𝑆𝑂4 3) (10 points) Determine the pH of the following aqueous solutions: a) 10-5.8 mol/L hydrogen ions (H+) b) 3×10-4 mg/L hydroxide ions (OH-) 4) (10 points) Suppose 25 mg of potassium hydroxide (KOH, a strong base) is dissolved in 500 mL of pure water. What is the pOH of the solution? What is the pH? 5) (15 points) Suppose 200 mg of formic acid (HCOOH, a weak acid) is dissolved in 1 L of pure water at equilibrium. The pKa value of formic acid is 3.74. What is the pH of the solution? (Note: the conjugate base of HCOOH is HCOO-) 6) (12.5 points) A glass of water is at equilibrium with the air at 25oC and 1.25 atm of atmospheric pressure. Air is made up of 0.04% carbon dioxide and CO2 has a Henry’s Law coefficient of 0.03363 mol*L-1*atm-1 at 25oC. Under these conditions, find the concentration of carbon dioxide in the glass of water. 7) (12.5 points) Solid zinc hydroxide, Zn(OH)2 is placed into a solution and has reached equilibrium with its ions: Zn(OH)2 ↔ Zn2+ + 2OH- Ksp = 10-15.5 Find the equilibrium concentration of hydroxide ions (in mg/L). (continued on the next page) Page 1 of 2 CE 347 – Spring 2021 8) (15 points) Drinking water utilities often add the chemical orthophosphate to drinking water to control the corrosion of metals such as lead in both the distribution system and plumbing within households and businesses. Addition of orthophosphate leads to the precipitation of lead and scaling of lead phosphate on the walls of pipes. During the Flint Water Crisis, the City of Flint, Michigan neglected to add orthophosphate to the water which led to the leaching of lead from pipes into the water, and in turn, the poisoning of a large portion of the city. If the EPA action limit for lead (Pb2+) in water is 15 ppb, and the Ksp for lead phosphate (Pb3(PO4)2) is 3 x 10-44, what concentration of phosphate (in mg/L) is needed in the system to maintain 15 ppb lead concentration? Pb3((PO4)2 will dissociate into Pb2+ and PO43-. Ignore all other metals and other forms of lead. 9) (15 points) A wastewater treatment plant uses an activated sludge process in a covered tank to remove organic matter from wastewater. This process consists of a tank of wastewater with pure oxygen being injected into the water to support growth of organisms that will consume the organic matter (we will learn more about this process later this semester). The composition of the gas in the tank’s headspace is in equilibrium with the wastewater and consists of 80% oxygen, 15% nitrogen, and 5% carbon dioxide. The gas is maintained at a total pressure of 3 atm. Determine the equilibrium concentration of oxygen and carbon dioxide in the aqueous phase of the covered activated sludge tank. Express your answer in mg/L and use the following Henry’s law constants: KH (O2) = 1.26 x 10-3 mol*L-1*atm-1 KH (CO2) = 3.41 x 10-2 mol*L-1*atm-1 Page 2 of 2
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