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!
The molar mass of an element is its atomic weight on
the periodic table expressed in grams per mole. For example, the molar
of carbon is 12.0 g/mol. The molar mass of a compound is the formula
in grams for one mole of that substance. Some examples are shown below:
(1 x 23.0) + (1 x 35.5) = 58.5 amu
(1 x 40.0) + (2 x 35.5) = 111 amu
(3 x 23.0) + (1 x 31.0) + (4 x 16.0) = 164 amu
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. For example, how many grams are in 3 moles of CaCl2?
Or, how many moles of CaCl2 is 55.5 g of CaCl2?
Mole Relationships in a Chemical Reaction
Mole relationships in a chemical reaction can be determined by looking at the balanced reaction equation as shown below for the reaction of aluminum (Al) with hydrochloric acid (HCl) to produce aluminum chloride (AlCl3) and hydrogen gas (H2):
A balanced reaction equation has numbers in front of each substance called coefficients. If there is no number in front of a substance, assume the coefficient to be 1. These coefficients tell us the ratio of how many elements or molecules of each substance will be consumed and produced in that chemical reaction. From the reaction equation above, we can see that for every 2 moles of Al, we will produce 2 moles of AlCl3. This mole relationship can also be used as a conversion factor. There are several conversion factors that we can derive from this balanced reaction equation:
Using the Mole Relationship as a Conversion Factor
The mole to mole relationships or equalities can be used as a conversion factor between moles of one substance to moles of another substance in the same chemical reaction. For example, if you started with 1.0 mole of Al in the reaction above, how many moles of H2 gas would be produced?
Or, if you want to produce 4.0 moles of AlCl3, how much Al would you need to start with?
In this experiment, you will be reacting sodium bicarbonate (NaHCO3) 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 bicarbonate. 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%
|a.||Mass of test tube and boiling chip|
|b.||Mass of test tube and boiling chip and NaHCO3|
|c.||Mass of NaHCO3 (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)||
Use your initial mass of NaHCO3 to calculate the number of moles of NaHCO3 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 NaHCO3 to NaCl to be? Do your results agree with this?
What was your percent yield of NaCl? List some possible reasons for
your yield to be lower (or higher) than 100%.