Cliffs AP Chemistry, 3rd Edition

Experiment 5: Determination of the Molar Volume of a Gas

Background: Avogadro’s law (V 1 n 2 = V 2 n 1 ), where moles, n MW grams mole(/)
grams
= expresses the

relationship between molar mass, the actual mass and the number of moles of a gas. The molar

volume of a gas at STP, VSTP

• is equal to the volume of the gas measured at STP divided by the

number of moles; VSTP=VnSTP

• . Dalton’s Law of Partial Pressure (Ptotal= P 1 + P 2 + P 3 +... ) and

the derivation, Pi=nntotali $Ptotalwill also be used in this experiment to predict the volume occu-

pied by one mole of hydrogen gas at STP.

Scenario: A student cut a 4.60 cm piece of pure magnesium ribbon. The student then prepared
a gas collecting tube with HCl at the bottom and water at the top and placed the magnesium in
a copper cage into the water end. The tube was then inverted (see Figure 1). The HCl being
more dense flowed down the tube and then reacted with the magnesium, producing bubbles of
gas. After all of the magnesium had reacted, the student then transferred the gas collecting tube
carefully to a large cylinder filled with water and adjusted the gas collecting tube so that the
water level in the tube was even with the water level in the cylinder. The student then read the
volume of the gas in the collecting tube as 40.44 cm^3. The temperature of the water was
26.3°C and the air pressure of the room was 743 mm of Hg.

Figure 1

(a)

(b)

(c)

600 ml

Part III: AP Chemistry Laboratory Experiments