In order to solve stoichiometric problems of chemical equation you need to know what does a balance chemical equation mean. Let us have an example in the combustion reaction of carbon monoxide.
The equation above means that 2 moles of CO reacts with 1 mole of O2 to produce 2 moles of CO2. It can also be interpreted by using the number of molecules. This also means that when 2 molecules of CO react with one molecule of O2, there are 2 molecules of CO2 produced.
Based from the given balanced equation you can already predict the amount of either products or reactants depending on the given. For example, if there are 3 moles of CO how much O2 is needed to produced CO2?
Now to solve the problem you have to identify the relationship that you can use in solving the problem. Since the question is asking the amount of O2 given the CO, you need to determine the ratio between O2 and CO.
To solve for the problem above we will use the relationship that 2 moles of CO needs 1 mole of O2 to produced CO2. There is 2:1 ratio between CO to O2. Using the factor label method in solving the problem:
What about the amount of moles of CO2 produced? You will now use the relationship either CO to CO2 or O2 to CO2. The ratio of CO to CO2 in balanced equation is 2:2 therefore, 3 moles of CO will also yield 3 moles of CO2.
The problem above is an example of mole to mole calculation. A calculation wherein the given is number of mole and asking the number of mole also.
Another type of stoichiometric calculation is mass to mass calculation. In this type of stoichiometry the given is mass and what is unknown is also mass. For example, using the same equation above, the reaction between CO to O2 to form CO2. How much CO2 will be produced when 10.5 g of CO is added to O2 to form CO2? So the above problem needs first the conversion of mass to no. moles and vice versa which means that the molar mass of the compound will be used.
The above conversion step can be used to solve the problem. Since the mass of CO is given, we need to calculate first the no. of moles of CO, and mole to mole conversion of CO to CO2 and grams or mass of CO2 can be calculated using the molar mass of the CO2.
Next is to solve the moles of CO2
And to calculate the mass of CO2, we need to use the molar mass of CO2 which is 44 g.
So the mass of CO2 is 16.6 g that can be produced from 10.5 g of CO.
We can combine the three calculation in one step only,
Sample Problem 1
If 200 g of methanol are used up in a combustion process , what is the mass of CO2 produced?
The balanced equation of the reaction of methanol with oxygen is
Solution:
In solving the problem we need to follow several steps.
Step 1. Convert mass of methanol to no. of moles. To convert mass to no. of moles, molar mass of methanol should be calculated first. Molar mass of CH3OH = 12 g + 4(1 g) + 16 g = 32 g/mol. (Note: In this blog the atomic mass used are always in whole number, meaning it is already rounded off).
Step 2. Calculate no. of mole of CO2 using the calculated mole of CH3OH using the balanced equation. From the balanced equation, in every 2 moles of CH3OH yields 2 moles of CO2. (2:2 mole ratio)
Step 3. Calculate the mass of CO2 using the mole of CO2 calculated above. Molar mass of CO2 is needed. Molar mass of CO2 = 12 + 2(16) = 44 g/mol.
The answer is 275 g of CO2.
Now if you want to have shorter solution, we can combine all steps
TRY THIS:
1. Limestone (CaCO3) is decomposed by heating to quicklime (CaO) and carbon dioxide. Calculate how many grams of quicklime can be produced from 350 g of limestone.
2. Nitrous oxide (N2O) is also called laughing gas. It can be prepared by the thermal decomposition of ammonium nitrate (NH4NO3). The other product is H2O.
a) Write a balanced equation for this reaction.
b) How many grams of N2O are formed if 0.46 mole of NH4NO3 is used in the reaction?
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