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## AP®︎/College Chemistry

### Course: AP®︎/College Chemistry>Unit 4

Lesson 4: Stoichiometry

# Worked example: Calculating amounts of reactants and products

A balanced chemical equation shows us the numerical relationships between each of the species involved in the chemical change. Using these numerical relationships (called mole ratios), we can convert between amounts of reactants and products for a given chemical reaction. Created by Sal Khan.

## Want to join the conversation?

• Why the molecular form of Oxygen is O2? Didn't learn that in previous lessons and so what is form of it if it is one Oxygen atom? •  Each oxygen atom fulfills its octet by bonding with another. Oxygen is part of the special group of elements whose atom's bond to each other called elemental diatomic molecules. The most common examples being H2, N2, O2, F2, and Cl2. Whenever you see oxygen or oxygen gas in a problem, you can assume they mean O2. You won't find monatomic oxygen in any stoichiometry problems.

Hope this helps.
• I don't understand the step where he says we have 0.833 mol O2. Can somebody explain that step to me? • The chemical formula for glucose is C6H12O6, which means for every molecule of glucose we'll have six oxygen atoms. Or in other words they will be related by a glucose to oxygen atom ratio of 1:6. So to determine how many moles of oxygen atoms are present in a sample of glucose, we simply need to multiple the moles of glucose by six. So if we have 0.139 mols of glucose, we will have 0.833 mols of oxygen (0.139 x 6 = 0.834, but accounting for sig figs it becomes 0.833).

Hope that helps.
• why are we using the amount of moles of oxygen to complete the second part of the question? • If we completely consume all the oxygen gas in the reaction, then we will have produced the maximum amount of products; carbon dioxide and water. So the amount of the products is directly related to how much oxygen gas we have to start with. The reason the moles of oxygen are equal to the moles of carbon dioxide and water is that they are related by a 6:6 ratio (or 1:1 ratio) in the chemical equation using their coefficients. So if we consume 0.833 moles of oxygen gas, then we’ll produce 0.833 moles of carbon dioxide and water as a result.

Hope that helps.
• at or so, I don't get why we multiply the mol of glucose by 6. • Help! () -

I also don't understand the relationship between the moles of O2 & the products CO2 & H20.

I've read through the explanations below and I think that the friction point for me is the fact that I don't understand how the mole ratio (6:1) transfers across the chemical equation to the products.

I understand that there are 6 moles of O2 for every mole of C6H12O6.

What I don't understand is how that is then related to the products (whose molar masses are then multiplied by the moles of O2).

Hoping someone can give me some guidance on this because I understood the whole lesson completely up until that point. • I thought you weren't supposed to round any numbers until the very end. So shouldn't the answer for the first part be 26.6 not 26.7. • Yes you are correct, Sal should not have rounded prematurely like that for the moles of glucose and should have rounded only at his final answer. So doing the same calculations Sal did, but only rounding until the very end should result in 26.6g. Still the same calculations and correct number of sig figs as before, but by avoiding the early rounding we avoid compounding the error which means a series of errors in your answers stemming from a single human error.

Hope that helps.
• At , why are the moles of O2 equal to the moles of water molecules? I see how the moles of O2 is equal to the moles of carbon dioxide since there is 1 mol of O2 in the carbon dioxide, but there is only 1 oxygen in the water molecule, so why is it equal? Thanks in advance.
(1 vote) • When Sal is using the moles of the chemicals for , he is using the coefficients of the balanced chemical equation at the top. When he says the moles of oxygen gas are equal to carbon dioxide and water, he means that in the chemical equation they all have a coefficient of 6. One mole of glucose reacts with 6 moles of oxygen to produce 6 moles of carbon dioxide and 6 moles of water. So when Sal finds the moles of glucose from the grams of glucose, he multiples the moles of glucose by 6 to find the moles of oxygen since they are related to each other by a 1:6 ratio in the balanced chemical equation. And to find moles of carbon dioxide from the moles of oxygen he uses a 6:6 ratio (really just a 1:1 ratio) from the chemical equation. Same idea for the water, the same 1:1 ratio.

The way I think you're interpreting it is that moles = atoms in each compound, but that's not what we're dealing with here. We're not referring to the chemical formulas of the chemicals at the end, just to the chemical equation at the top and the coefficients.

Hope that helps.
• I am confused as to why you use 0.833 for O2, CO2, and H20. • When Sal converts the mass of glucose to moles of glucose using its molar mass, he gets 0.139 mols. Glucose is related to the other chemicals by a 6:1 ratio using the balanced chemical equation. So the number of moles for oxygen, water, and carbon dioxide will be six times whatever the number of mols of glucose is (0.139 x 6 = 0.833).

Hope that helps.  