Fundamentals of Chemistry 1030
Another Mole Concept
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Purpose:

(1) Learn how to calculate the molar mass of any substance.
(2) Use the mole concept to convert grams to moles and moles to grams.
(3) React a known mass of Na2CO3 with excess HCl.
(4) Determine the mole relationship between reactant (Na2CO3) and product (NaCl).
 

The Mole Concept

The smallest drop of water that the naked eye can see is made up of billions and billions of water molecules. The "mole concept" is a tool that we can use to better grasp such astronomical numbers. A mole is a unit that is used to represent a very large number of atoms or molecules. One mole of any substance is 6.02 x 1023 (Avogadro's number) particles of that substance. Just as you would always assume that there are 12 eggs in a dozen, there will always be 6.02 x1023 particles in 1 mole of any substance. To give you an idea of how large of a number that really is...if all of the people now alive on the earth started counting Avogadro's number of peanuts at a rate of two peanuts per second, it would take approximately 2.6 million (2,6000,000) years. That's a lot of peanuts!

Molar Mass

The molar mass of an element is its atomic weight on the periodic table expressed in grams per mole. For example, the molar mass of carbon is 12.0 g/mol. The molar mass of a compound is the formula weight in grams for one mole of that substance. Some examples are shown below:
 
Molecular Formula
Formula Weight
Molar Mass
NaCl     Na               Cl 
(1 x 23.0) + (1 x 35.5) = 58.5 amu		 58.5 g/mol
CaCl2      Ca               Cl 
(1 x 40.0) + (2 x 35.5) = 111 amu		 111 g/mol
Na3PO4      Na              P                 O 
(3 x 23.0) + (1 x 31.0) + (4 x 16.0) = 164 amu		 164 g/mol
 

Using the Molar Mass as a Conversion Factor

The molar mass of an element or compound can be used as a conversion factor between grams and moles.

Today’s Experiment

In this experiment, you will be reacting sodium carbonate (Na2CO3) with hydrochloric acid (HCl) as shown below to produce sodium chloride, water and carbon dioxide:

At the beginning of the experiment, you will obtain the mass in grams of the sodium carbonate. Using the two conversion factors discussed above, you will be able to carry out the following conversion:

The grams of NaCl that you determine in this calculation is called the theoretical yield. At the end of the experiment, you will determine the mass in grams of the sodium chloride product and this is called the actual yield. According to the law of conservation of mass, the actual yield should be equal to the theoretical yield. However, due to human and experimental errors, it very seldom is. The percent yield is calculated using the following equation:

Percent yield = (Actual yield/Theoretical yield) x 100%
 

Laboratory Procedures

1. Thoroughly clean a large Pyrex test tube with soap and water.  Then rinse it with distilled water.  Fold a paper towel into a long thin strip and use this to remove as much of the water as possible.  Finally, use a Bunsen burner to 'flame dry' your glassware to ensure the removal of all moisture. CAUTION: The glassware will be very HOT - use your tongs when handling!

2.  Allow the test tube to cool to room temperature.  Then add a single boiling chip and measure the combined mass to the nearest 0.001 g. (Record your data in a table similar to the one below.)  With a scoopula, add approximately 1.500 g of sodium carbonate, Na2CO3, to the test tube and read the mass to the nearest   0.001 g.  Note: Do not try to measure exactly 1.500 g. Your measurement should be about 1.500 g, but record exactly.  For example, 1.442 g or 1.587 g are fine.

3. Obtain about 8 mL of 6M hydrochloric acid in your 10 mL graduated cylinder. CAUTION:  HCl causes acid burns - avoid contact with your skin.  Use a pasteur (disposable) pipette to slowly add the acid to the Na2CO3. Observe whether this reaaction is exothermic or endothermic. how does this compare to the previous experiment? When you add the acid to the Na2CO3, you should observe the evolution of gas (bubbles). Continue to add the acid slowly until the reaction is complete  (bubbling has stopped).  Do not add more acid than is needed.

4. Swirl the contents of the test tube to make sure the HCl has come in contact with all of the Na2CO3.  If any unreacted Na2CO3 remains (bubbles), add a few more drops of HCl to complete the reaction.

5.Clamp the test tube near the opening and at a 45° angle and gently heat the liquid over the Bunsen burner until it boils. Initially, use a small flame approximately 2-3 inches high. Take care to avoid loss of liquid from boiling over. If the liquid begins to splatter, remove the heat immediately.  Lower the flame and then continue to heat. Continue to dry the solid slowly until all moisture appears to have evaporated.

6. Allow the test tube to cool to room temperature and then measure its mass to the nearest 0.001 g.

7.Reheat the sample strongly for 2-3 minutes. Allow it to cool to room temperature and reweigh.
 
8.If this weight does not agree to within 0.001 g with the weight in Step 7, reheat and remeasure the mass until two consecutive weights are within 0.01 g of each other.  This is known as weighing to constant dryness and 'proves' that all of the water is gone.                                        Data Table  
a. Mass of  test tube and boiling chip  
b. Mass of test tube and boiling chip and Na2CO3  
c. Mass of Na2CO3 (b - a)  
d. Mass of  test tube and boiling chip NaCl (after first heating)  
e. Mass of  test tube and boiling chip NaCl (after second heating)  
f. Mass of NaCl (e - a)  

Issues to be addressed in your conclusion...

Use your initial mass of Na2CO3 to calculate the number of moles of Na2CO3 that were used in this reaction.

Use your final mass of NaCl to determine the number of moles of NaCl that were produced.

According to the balanced equation for this reaction, what would you expect the molar ratio of Na2CO3 to NaCl to be? Do your results agree with this?

What observations did you make that would lead you to conclude that the reaction of Na2CO3  with HCl is different than that of NaHCO3   ?

What was your percent yield of NaCl? List some possible reasons for your yield to be lower (or higher) than 100%.