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For 3Si + 2N2 giving Si3N4, which reactant limits when 2.00 g of Si (28.09 g/mol) and 1.50 g of N2 (28.02 g/mol) react?
ASilicon, since its molar ratio falls short
BNitrogen, since its mass is the smaller one
CSilicon, since it has the larger molar mass
DNitrogen, since its coefficient is smaller
Answer & Solution
Correct answer: A. Silicon, since its molar ratio falls short
1. Moles of Si equal 2.00 g divided by 28.09 g/mol, which is 0.0712 mol.
2. Moles of N2 equal 1.50 g divided by 28.02 g/mol, which is 0.0535 mol.
3. The supplied ratio is 0.0712 divided by 0.0535, that is 1.33 mol Si per mol N2.
4. The equation demands 3 divided by 2, that is 1.50 mol Si per mol N2.
5. Since 1.33 is below 1.50, silicon is short of the stoichiometric amount and limits the reaction.
6. Checking by product: 0.0712 divided by 3 gives 0.0237 mol Si3N4 from silicon.
7. Nitrogen gives 0.0535 divided by 2, which is 0.0268 mol, the larger figure, so nitrogen is in excess.
_Source: OpenStax Chemistry (CC BY 4.0), Ch 4 "Stoichiometry of Chemical Reactions", section 4.4 Reaction Yields_
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