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Topic: Have I done the calculations correctly?  (Read 1192 times)

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Offline shohaib712

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Have I done the calculations correctly?
« on: November 03, 2018, 07:02:44 PM »
One method used to inflate air bags in cars is to use nitrogen produced chemically from the decomposition of sodium azide. The sodium formed reacts with potassium nitrate to give more nitrogen.

2 NaN3(s) → 2 Na(s) + 3 N2(g)
10 Na(s) + 2 KNO3(s) → K2O(s) + 5 Na2O(s) + N2(g)

a) In what ratio (by mass) must the sodium azide and potassium nitrate be mixed in order that no metallic sodium remains after the reaction?
b) Calculate the total mass of the solid mixture needed to inflate a 60.0 dm3 air bag at room temperature and atmospheric pressure.

This how I would calculate the answers (don't know if its correct as I don't have a markscheme):

Multiply the first equation by 5 then calculate the mass of the compounds using the balancing numbers as the mole number (via stoicheomtery). That should tell you how much mass is needed for each reaction to work I believe.

b) work out the moles of nitrogen needed to produce that much gas via molar gas equation. should get 7/6 then divide it by 4 (= 7/24) to tell you the moles of n2 needed from the 2nd equation and multiply 7/24 by 3 (7/8) to tell you the moles of n2 for the 1st equation. Then using stoichiometry try to work out the mass of the solid reactants needed using those mole numbers... hope that works..

Offline Borek

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Re: Have I done the calculations correctly?
« Reply #1 on: November 04, 2018, 03:41:55 AM »
Multiply the first equation by 5 then calculate the mass of the compounds using the balancing numbers as the mole number (via stoicheomtery). That should tell you how much mass is needed for each reaction to work I believe.

Can be done this way, although you can try to just balance the overall reaction and work with a single reaction, not two.

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work out the moles of nitrogen needed to produce that much gas via molar gas equation

Not sure what you mean by molar gas equation, but yes, moles of nitrogen from the volume is the right starting point.

Quote
should get 7/6

No idea what these numbers are intended to be.
« Last Edit: November 04, 2018, 06:04:21 AM by Borek »
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