Well, you can calculate the percent yield using this formula: %yield = (actual yield/theoretical yield) x 100.
Step 1:
Start with a balanced chemical equation. A chemical equation describes the reactants (on the left side) reacting to form products (on the right side). Some problems will give you this equation, while others ask you to write it out yourself, such as for a word problem. Since atoms are not created or destroyed during a chemical reaction, each element should have the same number of atoms on the left and right side.
For example, oxygen and glucose can react to form carbon dioxide and water: 6O2+C6H12O6→6CO2→6H2O
Each side has exactly 6 carbon (C) atoms, 12 hydrogen (H) atoms, and 18 oxygen (O) atoms. The equation is balanced.
Step 2:
Calculate the molar mass of each reactant. Look up the molar mass of each atom in the compound, then add them together to find the molar mass of that compound. Do this for a single molecule of the compound.
For example, 1 molecule of oxygen (O2) contains 2 oxygen atoms.
Oxygen's molar mass is about 16 g/mol. (You can find a more precise value on a periodic table.)
2 oxygen atoms x 16 g/mol per atom = 32 g/mol of O2
The other reactant, glucose (C6H12O6) has a molar mass of (6 atoms C x 12 g C/mol) + (12 atoms H x 1 g H/mol) + (6 atoms O x 16 g O/mol) = 180 g/mol
Step 3:
Convert the amount of each reactant from grams to moles. Now it's time to look at the specific experiment you are studying. Write down the amounts of each reactant in grams. Divide this value by that compound's molar mass to convert the amount to moles.
For example, say you started with 40 grams of oxygen and 25 grams of glucose. 40 g O2/ (32g/mol)+1.25 moles of oxygen
25g C6H12O6/(180 g/mol) = approximately 0.139 moles of glucose
Step 4:
Find the ratio of your reactions. A mole is an exact number for the amount of a substance and it is equal to 6.022 times 10 to the 23rd power elementary entities, which could be atoms, ions, electrons, or molecules. You now know how many molecules of each reactant you started with. Divide the moles of 1 reactant with the moles of the other to find the ratio of the 2 molecules.
You started with 1.25 moles of oxygen and 0.139 moles of glucose. The ratio of oxygen to glucose molecules is 1.25 / 0.139 = 9.0. This means you started with 9 molecules of oxygen for every 1 molecule of glucose.
Step 5:
Find the ideal ratio for the reaction. Go back to the balanced equation you wrote down earlier. This balanced equation tells you the ideal ratio of molecules: if you use this ratio, both reactants will be used up at the same time.
The left side of the equation is 6O2+C6H12O6
The coefficients tell you there are 6 oxygen molecules and 1 glucose molecule. The ideal ratio for this reaction is 6 oxygen / 1 glucose = 6.0.
Make sure you list the reactants in the same order you did for the other ratio. If you use oxygen/glucose for 1 and glucose/oxygen for the other, your next result will be wrong.
Step 6 (last step):
Compare the ratios to find the limiting reactant. In a chemical reaction, 1 of the reactants gets used up before the others. The quantity of the product that is created in the reaction is limited by the reagent. Compare the 2 ratios you calculated to identify the limiting reactant:
If the actual ratio is greater than the ideal ratio, then you have more of the top reactant than you need. The bottom reactant in the ratio is the limiting reactant.
If the actual ratio is smaller than the ideal ratio, you don't have enough of the top reactant, so it is the limiting reactant.
In the example above, the actual ratio of oxygen/glucose (9.0) is greater than the ideal ratio (6.0). The bottom reactant, glucose, must be the limiting reactant.
Truly hope this helps Kayla,
~Mia.
