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### Topic: % w/w Problem  (Read 3514 times)

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#### ooooops

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• Mole Snacks: +0/-0 ##### % w/w Problem
« on: April 04, 2010, 12:58:55 AM »
I'm having trouble with the following question. Help would be greatly appreciated!

A sample of a mixture of KClO3 and KBr weighing 7.0950 g is heated in the
presence of a small amount of MnO2 which acts as a catalyst of the decomposition
reaction

2KClO3(s) -> 3O2(g) + 2KCl(s)

The O2(g) is collected in a container over water. The total volume and pressure of
the gas in the container was 865.5 mL and 741.5 mm Hg respectively at 27.2oC.
The vapour pressure of water at that temperature is 27.1 mm Hg. Assume that all
of the KClO3(s) decomposed. What was the %w/w of KClO3(s) in the original
mixture?

Thanks!

#### Matias Ekstrand ##### Re: % w/w Problem
« Reply #1 on: April 04, 2010, 01:19:23 AM »
What have you tried with so far?
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#### ooooops

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• Mole Snacks: +0/-0 ##### Re: % w/w Problem
« Reply #2 on: April 04, 2010, 11:28:05 AM »
What have you tried with so far?

I got the pressure of O2 by calculating 741.5 - 27.1 mm Hg = 714.4 mm Hg

Then i converted it to atm so i got 0.94 atm as the pressure of O2 in atm.
I then used the PV=nRT equation to get moles ("n")

(0.94 atm)(0.8655 L) = n(0.0821 Latm/Kmol)(300.2K)
0.033 mol = n

I dont really know what to do after finding the moles !

Thanks for your help #### Matias Ekstrand ##### Re: % w/w Problem
« Reply #3 on: April 04, 2010, 11:50:35 AM »
To reach the amount of substance of KClO3, you need to use the mole relationship to translate the amount of substance of O2 to the amount of substance KClO3 in the chemical equation. After this you calculate the mass of KClO3.

You wish to know a mass percentage. This means you need to divide the mass of KClO3 with the total mass of the mix.
« Last Edit: April 04, 2010, 12:01:05 PM by Matias Ekstrand »
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#### ooooops

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• Mole Snacks: +0/-0 ##### Re: % w/w Problem
« Reply #4 on: April 04, 2010, 12:35:47 PM »
To reach the amount of substance of KClO3, you need to use the mole relationship to translate the amount of substance of O2 to the amount of substance KClO3 in the chemical equation. After this you calculate the mass of KClO3.

You wish to know a mass percentage. This means you need to divide the mass of KClO3 with the total mass of the mix.

Thank you so much.. that really helped 