Molar Mass of CO2 Lab

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Vee lab Information
Earlier this year we determined that the relative mass of carbon dioxide was different than that of helium or oxygen. Now, with the Ideal Gas Law, we have the tool we need to determine the actual molar mass of this important gas. You will collect a sample of CO2 of known mass, volume, temperature, and pressure. How accurate and precise can you be?

Background:

You will make the carbon dioxide by reacting marble chips (calcium carbonate CaCO3) with hydrochloric acid (HCl). Note in the diagram the use of a thistle tube. The acid runs down the tube and reacts with the marble chips under water. The products are calcium chloride (which dissolves in the water), water, and carbon dioxide gas. The gas bubbles out of the solution and into the glass bend. The end of the thistle tube must be under water or the gas will escape out of the thistle tube. The gas then travels to a second flask for collection. The drying tube removes water vapor from the gas and allows us to collect dry CO2. Note the loose stopper. If this stopper were tight the gas pressure would increase in the second flask until it forced liquids back up the thistle tube. Because carbon dioxide is denser than air, the CO2 will flow into the bottom of the second flask and push the existing air out of the loose stopper. (Note: The glass tube entering this second flask is shown as short. Should it be short, or long (reaching near the bottom of the flask)? Be prepared to make a decision and tell me why!)

The drying tube contains calcium chloride. This compound is a desiccant: a material that absorbs water from the air It also contains an indicator that changes color from blue to pink in the presence of water.

Procedure:

  1. Get a 500 mL Florence flask with thistle tube assembly. Put about 25 g of marble chips and about 50 mL of water in the Florence flask. Then clamp it to a ringstand and attach the thistle tube assembly.
  2. Attach a filled drying tube to the glass bend exiting the Florence flask, and clamp the tube to the ringstand.
  3. Get a clean, dry 250 mL Florence flask, a one-hole stopper to fit, a short glass tube and a piece of latex tubing. Connect the glass tube to the stopper (use glycerin and a towel!). Attach this to the smaller Florence flask and mass the entire setup.
  4. Attach the latex tubing from this assembly to the drying tube, and clamp the smaller Florence flask to the ringstand.
  5. Use a volumetric pipet to put 10 mL of 6M hydrochloric acid into a small beaker. Bring a bottle of baking soda back to your table with the acid, in case of a spill. Check that the stopper entering the smaller Florence flask is loose, and have your teacher check your setup before going on.
  6. Slowly add the hydrochloric acid to the larger Florence flask through the thistle tube. Stop pouring if the acid backs up into the thistle tube. Generate CO2 for fifteen minutes. If the bubbling slows down, add another 10 mL of hydrochloric acid.
  7. After 15 minutes, fit the stopper tightly on the collecting flask and immediately remove the latex tubing from the flask.
  8. Mass the flask, stopper, and glass tube again.
  9. Get a thermometer. remove the stopper, quickly insert the thermometer, and read the temperature of the gas.
  10. Use water to determine the volume of the collecting flask.
  11. Measure room pressure.
  12. To dispose: (a) dump the liquid in the generating flask into the "waste acids" container, (b) rinse the marble chips with water and pour onto a paper towel to dry, (c) rinse out the generating flask and thistle tube assembly and put them on the drying rack, (d) return the drying tube still assembled, (e) disassemble the collecting flask parts and return.

 

Analysis:

  1. Calculate the molar mass of carbon dioxide.
  2. Share this with the class: determine average deviation and average error.
  3. Calculate your accumulated error.

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