My Math Forum determine mass of reactant not consumed

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 May 12th, 2018, 04:56 AM #1 Member   Joined: May 2015 From: Australia Posts: 78 Thanks: 7 determine mass of reactant not consumed 2Mg + O2 --> 2MgO If 6.00g of magnesium and 2.00g of oxygen are available for reaction, identify and determine the mass of the reactant that is not consumed. My attempted answer: n(Mg)= (6.00g) / (24.305g/mol) = 0.247mol n(O2)= (2.00g) / (32g/mol) = 0.0625mol Mg:MgO 2:2 n(MgO)=0.247 O2:MgO 1:2 n(MgO)= 0.0625 x 2 = 0.125mol Therefore, O2 is the limiting reagent and Mg is in excess. 0.247mol - 0.0625mol = 0.1845mol excess m(Mg) = n x Mr = 0.1845mol x 24.305g/mol = 4.48g excess The answer I got is 4.48g excess and thus this amount is not consumed. I'm not sure whether this is correct. My teacher got around 1g and he said he doesn't know whether it's correct as well. I tried putting this question into a 'stoichiometry calculator' I found online, and it also gave a different answer. I don't know who else to ask, so I hope someone on this forum would be able to help. =) Last edited by skipjack; May 12th, 2018 at 04:44 PM.
 May 12th, 2018, 04:43 PM #2 Global Moderator   Joined: Dec 2006 Posts: 21,027 Thanks: 2258 The 0.0625mol of O$_2$ reacts with 0.125mol of Mg, which has mass 0.125*24.305g = 3.04g so the mass of the remaining Mg is 6g - 3.04g = 2.96g. Thanks from pianist Last edited by skipjack; May 13th, 2018 at 02:51 AM. Reason: to correct the arithmetic.
 May 12th, 2018, 06:23 PM #3 Member   Joined: May 2015 From: Australia Posts: 78 Thanks: 7 Thanks skipjack, I understand it now. The hard part now is to explain this solution to my teacher...I'm not sure why he doesn't know what the actual solution is even though he's been teaching chemistry for a long time... when I calculate 0.122 x 24.305, I get 2.97g. Last edited by skipjack; May 13th, 2018 at 02:54 AM.
 May 13th, 2018, 02:53 AM #4 Global Moderator   Joined: Dec 2006 Posts: 21,027 Thanks: 2258 I'd mistyped 24.305 when doing the calculation. I've amended my reply and avoided a slight rounding error, so the answer is now 2.96g. Thanks from pianist

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