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# Chemistry calculation help! Watch

1. I am okay with part a) but I do not know how to calculate part b).. It would be really helpful if one of you could help me on this. I've got a mark scheme so it would be nice where these numbers come from (i.e. 2 and 72)..

Thanks!

16. Airbags, used as safety features in cars, contain sodium azide, NaN3. An airbag requires

a large volume of gas to be produced in a few milliseconds. The gas is produced in this

reaction:

2NaN3(s) -> 2Na(s) + 3N2(g) ????H is positive

When the airbag is fully inflated, 50 dm3 of nitrogen gas is produced.

(a) Calculate the number of molecules in 50 dm3 of nitrogen gas under these conditions.

[The Avogadro constant = 6.02 ???? 1023 mol–1. The molar volume of nitrogen gas under

the conditions in the airbag is 24 dm3 mol–1].

(b) Calculate the mass of sodium azide, NaN3, that would produce 50 dm3 of nitrogen

gas.

Mr = 23 + 42 = 65

Mass = (2 * 65 * 50/72) = 90g

But I don't know where 2 and 72 comes from in the mass equation.
2. looks like you times it by 2 because you have two moles, I havent looked at this much, but im guessing mass=moles*Mr, is this what you've done?
3. Times by two because you have 2NaN3

Could the 72 come because you've 3N2's, one mole of gas occupies 24dm^3
3*24=72?
4. (Original post by Kat_Heine)
Times by two because you have 2NaN3

Could the 72 come because you've 3N2's, one mole of gas occupies 24dm^3
3*24=72?
Ah, I see. Thank you so much!!

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Updated: December 16, 2010
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