Chapter 14 Laboratory: Gas Chemistry 261CUTIOA nS
The only real hazard in this lab is making a mess. (Dr. Mary
Chervenak says, “Yay!”) Wear splash goggles, gloves, and
protective clothing.z
My first thought was to use an inverted 100 mL graduated
cylinder as the gas collection vessel, which would have made it
easy to measure directly the volume of gas produced. A few quick
calculations convinced me to use a larger gas collection vessel. I
reasoned as follows:
- At standard temperature and pressure (STP), one mole of
gas occupies about 22.4 liters or 22,400 mL. - At full capacity, our 100 mL graduated cylinder could
therefore contain about [100 mL]/[22,400 mL/mol]
moles of gas, or about 0.0045 moles. - The gram-molecular mass of acetic acid is 60.05 g/mol,
so 0.0045 mol would be about 0.27 g of acetic acid. - If the vinegar has a mass percentage of 5%, 0.27 g of
acetic acid would be contained in about 5.36 g of vinegar. - In order to make sure I didn’t overshoot the capacity of
my graduated cylinder, I should reduce that amount by
25% or so, say to 4.00 g. - Using such small masses of reactants might well
introduce significant experimental error in weighing and
determining volume.
So I decided to use a larger gas-collection vessel. A 2-liter soft
drink bottle seemed perfect for the job. It has more than 20
times the capacity of our 100 mL graduated cylinder, is easy to
manipulate, and comes with a screw cap that makes it easy to
seal. (The reason for that will become obvious shortly.) The larger
size means we can use much larger amounts of the reactants,
which minimizes measuring errors.
FIGURE 14-5: My apparatus for gas generation and collection
POCEDURER
1.ou have not already done so, put on your splash If y
goggles, gloves, and protective clothing.
- Assemble the gas-generating bottle, as shown in Figure
14-5. (Be careful when you insert the glass tubing into the
stopper; lubricate it with glycerol or another lubricant,
and turn the tubing constantly as you slide it into the hole.
Use a heavy towel or similar item to protect your fingers if
the glass breaks.)- Determine the volume of the soft drink bottle by filling
the 100 mL graduated cylinder repeatedly with water and
emptying it into the bottle until the bottle is full to the
brim. (I cheated by using a 500 mL volumetric flask to put
2,000 mL of water into the bottle and then topping off the
bottle with the 100 mL graduated cylinder. My soft drink
bottle held 2,128.4 mL when filled to the brim.) Record
the volume in liters as accurately as possible on line A of
Table 14-4. - Without spilling any of the water from the bottle, replace
the cap and set the bottle aside. - Place the 100 mL graduated cylinder on your balance,
tare the balance, and weigh 75.00 g of vinegar. Record the
mass of the vinegar to 0.01 g on line B of Table 14-4. - Transfer the vinegar to the gas-generating bottle. To
make sure you’ve done a quantitative transfer, rinse the
graduated cylinder two or three times with several mL
of water and transfer the rinse water to the gas-
generating bottle. - Place a quarter-sheet of paper towel on the balance,
tare the balance, and weigh out about 7.5 g of sodium
hydrogen carbonate. The exact amount is not
critical, as long as the sodium hydrogen carbonate is
stoichiometrically in excess relative to the amount of
acetic acid. - Center the sodium hydrogen carbonate on the paper
towel, and roll up the paper towel such that several layers
of paper towel cover the chemical. Secure the package
with a rubber band or small piece of adhesive tape. - Place the soft drink bottle inverted in the plastic tub,
which should be about 80% full of water at room
temperature. With the mouth of the bottle submerged,
remove the cap from the bottle and lean the bottle
against the corner of the tub. (If the bottle won’t stay
put, you can either hold it in place or use a mouse pad or
similar nonslip item to prevent the mouth of the bottle
from sliding on the bottom of the tub. Alternatively, put a
brick in the tub and use it to keep the bottle upright.)
Insert the free end of the plastic tubing well up into
the bottle.