2. Sewage exists a force main and enters a gravity sewer**. At the exit of the f

Question

2. Sewage exists a force main and enters a gravity sewer**. At the exit of the force main all sulfur in the sewage is found as sulfide (H2S or HS-)*. When the sewage enters the gravity sewer it is exposed to oxygen that may cause the sulfide to transform to sulfate (SO42-). Assume that the pH is 7, the total sulfur concentration of the sewage where the force main meets the gravity sewer is 2.5x10-6M, and the sewage flow is 1.00x107liter/day. Hydrogen sulfide is a weak acid. The pKa for it’s first dissociation constant (H2S ó HS- + H+) is 7.0

A. Write a balanced equation for the oxidation of sulfide to sulfate

B. How much oxygen is required to transform all of the sulfide in a liter of sewage to sulfate?

C. How much limestone (CaCO3) would have to dissolve each day to keep the alkalinity of the sewage constant? i.e., neutralize the H+ generated by the oxidation of sulfide

D. What would the equilibrium [O2]aq be if the [SO42-]/[HS-] ratio is 1000?

*The S2- ion may also be present but its concentration will be negligible at allrelevant pHs and can be ignored

**In a force main the water/sewage is pumped and the fluid fills the whole pipe. Gravity sewers slope downhill so no pumping is needed. The sewers flow partially full; i.e., there is air above the water/sewage

Explanation / Answer

I have to write a balanced equation for the decompostion of solid copper (ii) sulfate to form the gaseous sulfur trioxide and solid coper (ii) oxide.



CuSO4 >>> SO3 + CuO
Na2SO4 ==> Na2O + SO3
The equation is balanced.
Place a delta sign or write the word "heat" over the arrow. OR it can br done this way.
Na2SO4 + heat ==> Na2O + SO3
Some teachers like to write the state (s for solid, g for gas and l for liquid).
Na2SO4(s) + heat ==> Na2O(s) + SO3(g)

Dissolved oxygen (O2) This is determined by two methods. One uses an "oxygen selective" electrode and is an electrochemical method which can be taken out of the laboratory. The other is based on the Winkler chemical method which involves a series of ionic and redox reactions which result in the formation of iodine at a concentration proportional to the initial concentration of dissolved oxygen in the sample. The amount of iodine is then determined using a redox titration. Dissolved oxygen is essential for any aerobic biochemical activity to occur, thus its levels are a useful indicator of biochemical activity. The process involves dissolving MnSO4, NaOH and NaI in the sample to be tested. These are then involved in a series of reactions. Firstly, a manganese hydroxide precipitate forms: MnSO4 + 2NaOH Mn(OH)2

This is then oxidised by the dissoved oxygen to a MnO2 precipitate: 2Mn(OH)2 + O2 2MnO2 + 2H2O The solution is then acidified to dissolve the precipitate so that it can react with the iodide in solution to form iodine: MnO2 + 4I- + 4H+ Mn2+ + 2I2 + 2H2O Thus overall the manganese has simply acted as an oxygen carrier, and the reaction occurring has been: 4OH- + O2 + 8I- + 8H+ 6H2O + 4I2 The concentration of the iodine is then determined by titration with Na2S2O3. Before titration the solution is yellow-brown (the colour of aqueous iodine). When this colour is almost too pale to be detected, a small amount of starch is added because starch forms a dark blue-black complex with iodine. When the blue-black colour disappears, end-point has been reached. It is much easier to see when the starch complex has disappeared than to see when the pale straw colour of the iodine is gone. The equation for this redox titration is: S2O3 2- + 5H2O + 2I2 2SO4 2- + 10H+ + 4IThus 1O2 = 4I2 = 2S2O3 2-CuCO3 -> CuO + CO2

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