there are 4 questions, general chemistry

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February 7, 2021
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February 7, 2021

there are 4 questions, general chemistry

Problem 1

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

2

to become supercritical?

f) Draw a complete Lewis structure for CO

2

, and use this to explain why it would

be useful as a cleaning solvent.

g) What would be the environmental advantages of CO

2

versus

perchloroethylene?

Problem 2

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

stream.

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

calcium hydroxide.

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?

Problem 3

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.

Problem 4

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?

 

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