Dry cleaners used to use the solvent perchloroethylene as the method of choice
for cleaning garments. It had several physical properties that made it a very
effective cleaning agent. Unfortunately, it was also an environmentally persistent
toxin, so its use was banned a number of years ago.
a) What is the formula of perchloroethylene (yes, you’ll look that one up!)?
b) Draw the complete Lewis structure for perchloroethylene.
c) Based on the Lewis structure, what property of this solvent would make it good
for cleaning clothing? (Hint: Think of the kinds of things that make clothes dirty.)
d) If the normal boiling point of perchloroethylene is 120.8°C and the vapor
pressure at 100°C is 400 mmHg, how much energy would it take to evaporate 55
gallons of the solvent? (Again, you’ll be searching for a few pieces of data here.)
Modern dry cleaners sometimes use supercritical fluid carbon dioxide as the
cleaning solvent of choice.
e) What are the minimum conditions needed for CO
to become supercritical?
f) Draw a complete Lewis structure for CO
, and use this to explain why it would
be useful as a cleaning solvent.
g) What would be the environmental advantages of CO
A qualitative test for the presence of carbon dioxide gas is the lime water test. A
saturated solution of calcium hydroxide is made by adding quick lime to a
container of water and allowing the reaction to come to equilibrium. The clear
supernantant is then poured off, and the gas is bubbled through that clear
solution. If a precipitate forms, then there was carbon dioxide present in the gas
a) Write the balanced chemical equation, including all states of the substances,
for the reaction between quick lime and water to form a saturated solution of
b) What is the molar concentration of the calcium hydroxide in the lime water?
c) Write the balanced equation for the reaction that occurs when carbon dioxide
is bubbled into lime water.
d) How many grams of carbon dioxide would it take to react with all of the
calcium hydroxide in 100. mL of lime water?
e) If the gas being tested is 5.0% (by volume) carbon dioxide, how many mL of
this gas at 35° C and 785 torr would it take to react with the calcium hydroxide in
100. mL of lime water?
I once was watching an episode of CSI, and there was a body dissolving in a
swimming pool. The investigators determined it was because the swimming pool
had been laced with potassium hydroxide. Human tissue needs a pH in excess of
13 to dissolve that quickly.
a) Assume the swimming pool was a rectangular swimming pool, 10 m long, 5 m
wide, and had a water depth of 2 m. How much potassium hydroxide would have
been dumped into the pool to get the necessary pH?
b) The clever team decided they would neutralize the base with a convenient acid
– vinegar. If the vinegar they used was the typical 3% found at a grocery store,
what volume would they need to reduce the alkalinity to pH 8?
Once I saw what the team was doing on the show, I had to quit watching, as it
offended my chemical sensibilities.
My wife hunts dead people – specifically, her ancestors (she’s a genealogist).
Gravestones are an excellent source of information, but historically they have
been made of limestone because of its ease of cutting and carving. Unfortunately
limestone dissolves in rain because of the acidity of the rain (slowly … but over a
couple hundred years it does wear away).
a) A major component of rain acidity is nitric acid formed from lightning
discharges. What is the reaction of the limestone with the acidic rain that eats
away at the stone?
b) If the pH of rain from a thunderstorm is 2.5, and 10 L of rain fall on a stone in a
storm, how much limestone can be dissolved for each storm? (Assume all of the
acid reacts with all of the limestone.)
c) If a gravestone has dimensions of 18 inches wide, 30 inches tall, and 2.0 inches
thick, how many rainstorms would it take to wear away enough to make the
carving unreadable if they were 0.20 inches deep?