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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: zoh899 on January 06, 2021, 06:15:31 AM

Title: Mole concept
Post by: zoh899 on January 06, 2021, 06:15:31 AM
he nitrates of calcium, strontium or barium are often added to firework mixtures to produce red or green flames. The equation for the decomposition of one such mixture is as follows. Sr(NO3) 2(s) + 3C(s) ⎯→ SrO(s) + N2(g) + 2CO2(g) + CO(g) Calculate the volume of gas given off (measured at room temperature and pressure) when a 10.0 g sample of this mixture decomposes. [Mr : Sr(NO3) 2, 211.6] .
Here's what I did :

10/211.6+32 = 0.0404

4:0.0404

1: x

i got x to be 0.0101 (these are the moles of strontium nitrate)

and then i used mole ratio to find the moles of the individual gases and i multiplied the moles of each individual gas by 24 to get the volume but this method seems to be incorrect as in the marking scheme they've done the following :

0.0404 x 4 = 0.162 moles

0.162 x 24 = 3.88 dm^3

Can someone please help
Title: Re: Mole concept
Post by: Borek on January 06, 2021, 07:22:23 AM
have you checked if the sum of volumes calculated your way is not identical with the result they got?
Title: Re: Mole concept
Post by: zoh899 on January 06, 2021, 07:28:54 AM
Quote from: Borek on January 06, 2021, 07:22:23 AM
have you checked if the sum of volumes calculated your way is not identical with the result they got?

Yes i checked it, according to my method, when i use 0.0101, i get the wrong volume, but if i use 0.0404 and do the whole mole ratio calculations for the individual gases, im getting the right answer, is it possible that none of the reactants is a limiting reagent, thats why we have to use the combined moles of the reactants?
Title: Re: Mole concept
Post by: mjc123 on January 06, 2021, 07:49:59 AM
If you are using 211.6 + 36 (not 32) as the effective MW of the mixture (Sr(NO3)2 + 3C), then 0.0404 is the number of moles of mixture. 1 mole of mixture contains 1 mole of strontium nitrate and 3 moles of carbon. Therefore the number of moles of Sr(NO3)2 is 0.0404, not 0.0101.
Title: Re: Mole concept
Post by: zoh899 on January 06, 2021, 08:12:38 AM
Quote from: mjc123 on January 06, 2021, 07:49:59 AM
If you are using 211.6 + 36 (not 32) as the effective MW of the mixture (Sr(NO3)2 + 3C), then 0.0404 is the number of moles of mixture. 1 mole of mixture contains 1 mole of strontium nitrate and 3 moles of carbon. Therefore the number of moles of Sr(NO3)2 is 0.0404, not 0.0101.

oh okay, tysm