ECIV 350 TEST QUESTIONS AND ANSWERS
(Lecture 3-5) A water contains two nitrogen species. The concentration of NH3 is 30
mg/L NH3, and the concentration of NO3 - is 5 mg/L NO3 - . What is the total nitrogen
concentration in units of mg N/L? - Answer -25.8 mg N/L
(Lecture 3-5) What pH is required to reduce a high concentration of dissolved Mg2+ to
43 mg/L? Ksp for the following reaction is 10-11.16. Mg(OH)2(s) Mg2+ + 2OH Ksp = 10-
11.16 - Answer -Ph=9.79
(Lecture 3-5) How long will it take the carbon monoxide (CO) concentration in a room to
decrease by 99% after the source of CO is removed and the windows are opened?
Assume the first-order rate constant for CO removal (due to dilution by incoming clean
air) is 1.2/hr. No chemical reaction is occurring. - Answer -2.69
(Hw#1) 3-1 from Textbook. How many grams of NaCl would you need to add to a 1 L
water sample (pH = 7) so the ionic strength equaled 0.1M? - Answer -5.8g
What percentage of total ammonia (i.e., NH3 + NH4 + ) is present as NH3 at pH 7? pKa
for NH4 = 9.3. NH4 + NH3 + H+ - Answer -05%
(Hw#2) 4-6 from Textbook. Poorly treated municipal wastewater is discharged to a
stream. The river flow rate upstream of the discharge point is Qu/s = 8.7 m3 /s. The
discharge occurs at a flow of Qd = 0.9 m3 /s and has a BOD concentration of 50.0
mg/L. Assuming that the upstream BOD concentration is negligible. (a) What is the BOD
concentration just downstream of the discharge point? (b) If the stream has a cross-
sectional area of 10 m2 , what would the BOD concentration be 50 km downstream?
(BOD is removed with a first-order decay rate constant equal to 0.20/day.) - Answer -a)
Cd=4.7 mg/L
V=0.96 m/s
θ=0.6 days
b)Cl=4.20mg/L
(Hw#2) 4-10 from Textbook. In the winter, a stream flows at 10 m3 /s and receives
discharge from a pipe that contains road runoff. The pipe has a flow of 5 m3 /s. The
stream's chloride concentration just upstream of the pipe's discharge is 12 mg/L, and
the runoff pipe's discharge has a chloride concentration of 40 mg/L. Chloride is a
conservative substance. (a) Does wintertime salt usage on the road elevate the
downstream chloride concentration above 20 mg/L? (b) What is the maximum daily
mass of chloride (metric tons/day) that can be discharged through the road runoff pipe
without exceeding the water quality standard? - Answer -a) 21.33 mg/L
b) 16.0 tons/day
