CEE 670 |
Fall 2011 |
Problem 26. A glass vial contains 0.5%
potassium by weight. The natural
abundance of 40K, which decays by β-emission
is 0.00118%, and its half-life is 1.28x109 years. If the vial weighs 15g, calculate the
expected rate of β- production by the vial in disintegrations
per minute or dpm. (problem
1 in Chapter 2 of Brezonik; pg.102).
Problem 27. Using the kinetic plot and
data for the hydrolysis of Benzyl Chloride as presented in class (refer to posted slides on
website[1]), determine the following:
a. The expected concentration
of benzyl chloride after 72 hours (@25°C), when the initial concentration
was 33µg/L.
b. The expected concentration
of benzyl chloride after 72 hours (@25°C), when the initial
concentration was 7.9µg/L.
c. The expected concentration
of benzyl chloride after 72 hours (@0.1°C), when the initial
concentration was 33µg/L.
Problem 28. It has been known for some
time that concentrated solutions of aqueous chlorine lose strength with
time. Accompanying this is the
accumulation of chlorate ion. Early
chemical studies have determined this to be a second order reaction in hypochlorite. The stoichiometry is as follows:
Bolyard and co-workers [1992, Env. Sci. Technol. 26(8)1663-1665] found that chlorate
is being inadvertently added to drinking water when aqueous chlorine is used as
a disinfectant. Data collected at 14
sites showed that raw water chlorate levels were 0.02 mg/L or below. However, finished water levels were as high
as 0.66 mg/L, apparently due to contamination from the chlorine stocks. Gordon and Adam [Water Disinfection News,
5:1, Novatek] have subsequently studied this reaction. They determined that a 2.776 M chlorine stock
solution (15.89% FAC, d=1.239 g/mL) has a 61.7 day half-life at 25°C and pH 13.
Based on this information answer the following
questions.
a. What is the 2nd
order rate constant for this reaction in units of M-1s-1?
b. Calculate the concentration
of chlorate in the stock after 40 days holding time at 25°C
c. What is the dosed
concentration (mg/L) of chlorate if sufficient stock (aged 40 days at 25°C) is added to a finished drinking water to
achieve a free chlorine dose of 2.5 mg/L?
d. Repeat "c", but
assume that the chlorine stock was diluted by a factor of 4 prior to holding
for 40 days. Assume that dilution does
not change the value of the rate constant [we will find out later that it will
change a bit due to the change in ionic strength].
e. Consider an MCLG (maximum
contaminant level goal) of 0.2 mg/L for chlorate. Determine the amount of dilution of the
chlorine stock, which will just allow compliance after 40 days holding time at
25°C.
Assigned: 17 Nov 11
Due: 29 Nov 11