Homework Problem M.7 (b)

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Homework Problem M.7 (b)

Postby GFolk_1D » Sat Sep 28, 2019 3:42 pm

Solid boron can be extracted from soli boron oxide by reaction with magnesium metal at a high temperature. A second product is solid magnesium oxide. (b) What mass of boron can be produced when 1.25kg or boron oxide is heated with 125kg of magnesium?

I converted the kg to g and then solved for the mol of each compound (B2O3- 0.00178 mol & Mg- 0.00514 mol). Then by comparing the ratios I concluded that Mg was the limiting factor. Is this the correct way to begin this problem? I am confused about how to continue.

Thank you!

Jessica Tran_3K
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Re: Homework Problem M.7 (b)

Postby Jessica Tran_3K » Sat Sep 28, 2019 4:09 pm

Hi! Yes, Mg is the limiting reactant. Now that you've concluded that, you can use dimensional analysis to convert the amount of mols you calculated for Mg into Boron using the mole ratio (2 mols of B for 3 mols of Mg) and then the molar mass to convert mols to grams (for the mass of Boron).

Hiba Alnajjar_2C
Posts: 108
Joined: Fri Aug 09, 2019 12:17 am

Re: Homework Problem M.7 (b)

Postby Hiba Alnajjar_2C » Sun Sep 29, 2019 12:38 pm

Jessica's method of using dimensional analysis to find the maximum mass of Boron is exactly correct. However, another way to both determine what the limiting reactant is (through stoichiometry) and the mass of Boron would be to perform dimensional analysis given the two masses of B2O3 and Mg. Essentially, you could convert both to moles, multiply that by the mole ratio of B/B2O3 and B/Mg, and finally convert from moles of B to grams--you would just be doing it for both reactants. Then, you would compare both final masses of Boron and whichever results in a lower quantity for the mass of Boron would be the limiting reactant. The nice thing about this method is that, by the end of it, you already have the maximum mass of Boron, which you found while simultaneously determining which reactant was the limiting reactant. Hope this helps & please feel free to ask follow-up questions if something I stated didn't make sense!

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