Plan for Tues/Wed, 28/29 Oct 08
• Turn in Exp 4 Formal Report and Exp 5 Pre-lab
• Today: Exp 5, Gas Laws
• Purpose:
– To employ the Ideal Gas Law and Dalton’s Law of partial
pressures to determine the number of moles of CO2(g) produced
in the reaction:
HCl(aq) + NaHCO3(s)  H2O(l) + CO2(g) + NaCl(aq)
– To determine the mass-% of sodium bicarbonate in a sample of
the antacid Alka Seltzer.
– To use these techniques to determine the mass-% of calcium
carbonate in an unknown sample.
2 HCl(aq) + CaCO3(s)  H2O(l) + CO2(g) + CaCl2(aq)
Tips for success
TECHNIQUE
•
DO NOT let the level of water in your buret fall below the markings!!
•
MAKE SURE your experimental set-up is free of leaks!!
•
MAKE SURE the water levels in your buret and leveling bulb are equal before the reaction occurs.
•
DO NOT disconnect any tubing in your set up while there is still water in your buret. Pour the water out
through the leveling bulb.
PROCEDURE
•
Obtain two consistent trials, within 5-10% of each other, not within 5-10% of the expected mass-% of
NaHCO3 in Alka Seltzer
WASTE
•
Acetone rinse should be disposed of in the liquid waste container.
•
Excess HCl(aq) may be poured down the drain and followed with 10 -20
•
NEVER put a spatula, etc into a reagent bottle. NEVER return unused reagent to the reagent bottle.
Any unused reagent must be placed in the waste container.
•
DO NOT overfill the liquid waste containers. Let me know if they are getting too full and I will replace
them.
SAFETY
•
HCl(aq) can cause chemical burns!! BE CAREFUL!! Gloves are available at the back of the lab. Inform
me or the lab staff of any acid spills
•
Be sure to wash and dry your lab bench to remove all traces of any spilled chemicals.
•
Wash your hands with soap and water before you leave lab.
Vapor Pressure
• Some molecules at the surface
of a sample of liquid have
enough kinetic energy to enter
the vapor phase, even if the
liquid is not boiling.
• This is why a glass of water left
open will eventually completely
evaporate.
• This is an example of vapor
pressure, the magnitude of
which depends on the
temperature of the liquid.
• As T goes up, Pvap also goes up.
Collecting gases over water
• Gaseous products of
reactions are often measured
by the displacement of water.
• The pressure of the gas
inside the collection tube is a
sum of the pressures of the
product gas and of the water:
Ptotal = Pproduct + PH2O
• If you know the total P
(usually atmospheric P), and
PH2O, you can find Pproduct.
Determining the total pressure
• Your experimental set-up is similar to a manometer.
Pgas < Patm
h
Pgas > Patm
h
If the level of the mercury in
each side of the tube were
the same, the trapped gas
would be at the same
pressure as Patm.
In your set-up, you are
using DI water rather than
Hg, and you can manually
adjust the water level by
moving the leveling bulb up
or down.
Determining Moles of CO2(g)
• From your experimental set-up, you will obtain the volume of CO2(g)
produced.
• Using this value, the atmospheric pressure, the temperature of the
water, and the vapor pressure of water at that temperature, you can
calculate the number of moles of CO2(g).
Temperature of
the water
PCO2 = Patm – PH2O
PCO2 V = nCO2 RT
Displacement of
water in buret
What you want
Determining Mass-% NaHCO3 in Alka Seltzer
• Using the moles of CO2 you just determined and the chemical
equation below, you can find the mass of NaHCO3(s) in your original
sample of Alka Seltzer.
HCl(aq) + NaHCO3 (s)  H2O(l) + CO2 (g) + NaCl(aq)
y g NaHCO3 = x mol CO2
1 NaHCO3
84.01 g NaHCO3
1 CO2
1 mol NaHCO3
• Mass Percent:
Mass-% NaHCO3 =
y g NaHCO3
Mass of your Alka Seltzer sample
X 100