What is the percentage of KClO3 in a mixture after removing oxygen?

In summary: Find the mass of that number of moles of KClO3You would use the molar mass of KClO3 to find the number of moles of KClO3.
  • #1
brbrett
35
0

Homework Statement


A 16.00g sample of a mixture of KClO3 and KCl is heated until all the oxgen has been removed from the sample. The product entirely KCl (no O), has a total mass of 9.00g. What is the percentage of KClO3 present in the original mixture?

16.00g mixture
9.00g mixture without oxygen
3.00g oxygen without mixture

Homework Equations


Percent Composition (percent = mass of element/mass of compound)
Various mole equations (eg. moles to mass)

The Attempt at a Solution


I received this question on a test awhile back and, while I have received the answer and the steps to reach it, I cannot understand why it is that it is done that way. From what I can see, we do not need to worry about percent composition of the mixture until the end. At the moment, we do not have KCl by itself, as O3 is bonded to it.

I think I need to find the mass of KCl in the original mixture, and so to do that, I chose to calculate the moles in KCl and O.

9.00gKCl/(75g/mol) = 0.12molKCl
3.00gO/(16g/mol)= 0.1875mol O
By this point, I had already figured that this was likely going nowhere, as I still did not know the amount of KCl in KClO3 by itself, but rather just the moles of KCLO3+KCL without the oxygen.

------------------------------
Next, is my teacher's solution.
gKClO3 = 3.00gO x (1.22gKClO3/48.9gO) = 7.6625
% = (7.6625/12.00) x 100% = 63.9%

I understand the percent part, but what I do not understand is why 1.22gKClO3 is divided by 48.0gO. If I could have insight onto that, then I would be most appreciative.

Thanks for reading my wall of text!
 
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  • #2
The teacher's solution glosses over some intermediate steps, so it's probably not as clear as it should be. Here are the steps I would take:

1. Find the number of moles of oxygen atoms in 3g. (0.1875 moles as you calculated)
2. Find how many moles of KClO3 can be created from 0.1875 moles of oxygen atoms.
3. Find the mass of that number of moles of KClO3

If you compress all of these 3 steps into one step, you get your teacher's solution.
 
  • #3
brbrett said:

Homework Statement


A 16.00g sample of a mixture of KClO3 and KCl is heated until all the oxgen has been removed from the sample. The product entirely KCl (no O), has a total mass of 9.00g. What is the percentage of KClO3 present in the original mixture?

12.00g mixture
9.00g mixture without oxygen
3.00g oxygen without mixture
I'm confused. Is the mass of the sample 16.00 g or 12.00 g?

Homework Equations


Percent Composition (percent = mass of element/mass of compound)
Various mole equations (eg. moles to mass)

The Attempt at a Solution


I received this question on a test awhile back and, while I have received the answer and the steps to reach it, I cannot understand why it is that it is done that way. From what I can see, we do not need to worry about percent composition of the mixture until the end. At the moment, we do not have KCl by itself, as O3 is bonded to it.

I think I need to find the mass of KCl in the original mixture, and so to do that, I chose to calculate the moles in KCl and O.

9.00gKCl/(75g/mol) = 0.12molKCl
3.00gO/(16g/mol)= 0.1875mol O
By this point, I had already figured that this was likely going nowhere, as I still did not know the amount of KCl in KClO3 by itself, but rather just the moles of KCLO3+KCL without the oxygen.

------------------------------
Next, is my teacher's solution.
gKClO3 = 3.00gO x (1.22gKClO3/48.9gO) = 7.6625
% = (7.6625/12.00) x 100% = 63.9%

I understand the percent part, but what I do not understand is why 122.6gKClO3 is divided by 48.0gO. If I could have insight onto that, then I would be most appreciative.

I'm confused again. You write 1.22 g on one line, and then you mention 122.6 g on the next line.

You should proofread your posts carefully before submitting them.
 
  • #4
Could I multiply the mass of KClO3 by 0.1875 molesO to find the number of moles of KClO3? I'm not entirely sure
SteamKing said:
I'm confused. Is the mass of the sample 16.00 g or 12.00 g?
I'm confused again. You write 1.22 g on one line, and then you mention 122.6 g on the next line.

You should proofread your posts carefully before submitting them.
You will have to forgive me. The information in the original question is the correct information. Anything else is my own error.
 
  • #5
Ygggdrasil said:
2. Find how many moles of KClO3 can be created from 0.1875 moles of oxygen atoms.
I'm not entirely certain of how to find the number of moles KClO3 has using the number of moles of Oxygen atoms. Would multiplying 0.1875molO by the total mass of the mixture give me the moles of KClO? (Though I do not believe that to be correct...)
 
  • #6
brbrett said:
I'm not entirely certain of how to find the number of moles KClO3 has using the number of moles of Oxygen atoms. Would multiplying 0.1875molO by the total mass of the mixture give me the moles of KClO? (Though I do not believe that to be correct...)
If you have 3 moles of oxygen atoms, how many moles of KClO3 do you have? Always look at the chemical formula.
 
  • #7
I have one mole of KClO3 then, with 1 mole of both K and Cl?
(39+35.5)*0.1875 = 13.96875gKCl

However, this number is more than double the number my teacher had gotten.
 
  • #8
brbrett said:
I have one mole of KClO3 then, with 1 mole of both K and Cl?
(39+35.5)*0.1875 = 13.96875gKCl

However, this number is more than double the number my teacher had gotten.
If you have one mole of KClO3, how many moles of oxygen do you have?
 
  • #9
SteamKing said:
If you have one mole of KClO3, how many moles of oxygen do you have?
3
 
  • #10
brbrett said:
3
Now, work backwards. You have 0.1875 moles of oxygen. How many moles of KClO3 would 0.1875 moles of oxygen make?
 
  • #11
0.0625 moles?
 
  • #12
brbrett said:
0.0625 moles?
Yes, this seems reasonable. What is the mass of this number of moles of KClO3 ?
 
  • #13
0.0625*122.55 = 7.659375g
I see what I needed to do now. I guess I had forgotten that a mole is just a representative unit.
% = (7.659375/12.00) x 100% = 63.828125 = 63.8%
The number is slightly off, but that is just error.
Thanks for your help!
 

1. What is a mole?

A mole is a unit of measurement used in chemistry to express the amount of a substance. It is defined as the amount of a substance that contains the same number of particles (atoms, molecules, or other particles) as there are atoms in 12 grams of carbon-12. This number is known as Avogadro's number, which is approximately 6.022 x 10^23.

2. How is the molar mass of a substance calculated?

The molar mass of a substance is calculated by adding up the atomic masses of all the atoms in the molecule. The atomic mass can be found on the periodic table and is measured in atomic mass units (amu). For example, the molar mass of water (H2O) would be calculated by adding the atomic mass of two hydrogen atoms (2 x 1.008 amu) and one oxygen atom (1 x 16.00 amu), resulting in a molar mass of 18.02 grams per mole.

3. What is percent composition?

Percent composition is the percentage by mass of each element in a compound. It is calculated by dividing the mass of each element in the compound by the total mass of the compound and then multiplying by 100%. This can be used to determine the empirical formula of a compound.

4. How do you calculate percent composition?

To calculate percent composition, you first need to determine the molar mass of the compound. Then, divide the molar mass of each element by the molar mass of the compound and multiply by 100%. For example, if a compound has a molar mass of 60 g/mol and contains 20 grams of oxygen and 40 grams of nitrogen, the percent composition of oxygen would be (20 g/60 g) x 100% = 33.3% and the percent composition of nitrogen would be (40 g/60 g) x 100% = 66.7%.

5. Why is percent composition important in chemistry?

Percent composition is important in chemistry because it allows us to determine the amount of each element present in a compound. This information can be used to determine the empirical formula of the compound, which is important for understanding the chemical properties and reactions of the compound. Percent composition can also be used to check the purity of a substance and to calculate the mass or volume of a specific element in a chemical reaction.

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