CHY103-4-2 Skeleton.pdf

# 2to decide which of the reagents is limiting a divide

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(2)To decide which of the reagents is LIMITING: a. Divide the number of moles of each reagent by its coefficient in the BALANCED equation. b. INSPECT the numbers so obtained; the lesser value indicates the LIMITING REAGENT - and the moles of products will depend ONLY on the moles of this reactant - all the others being in EXCESS.

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Example: Phosphorus, P 4 is made by the following reaction: 2 Ca 3 (PO 4 ) 2 + 6 SiO 2 + 5C P 4 + 6 CaSiO 3 + 5 CO 2 Calculate the maximum mass of P 4 ( M (P 4 ) = 123.9 g mol -1 ) that can be made by reacting the following: 6.00 g Ca 3 (PO 4 ) 2 M (Ca 3 (PO 4 ) 2 ) = 310 g mol -1 4.00 g SiO 2 M (SiO 2 ) = 60.1 g mol -1 1.00 g C M (C) = 12.01 g mol -1 The first step is to recognize that we have a limiting reagent problem, then decide which is the limiting reagent.
Substance coefficient moles Mole/coefficient ratio Ca 3 (PO 4 ) 2 SiO 2 C 0.0194 0.0666 0.0833 What is the limiting reagent? 5 6 2

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Since Ca 3 (PO 4 ) 2 is LIMITING we use it to determine the number of moles of P 4 produced. Finally, we determine the mass of P 4 produced: Mass P 4 = 0.00970 mol x 123.9 g mol -1 = 1.20 g P 4 Follow up question: How many grams of carbon are remaining?
Yields There are three types to be aware of: 1. The __________ yield is what we predict or calculate we would obtain if the reaction went perfectly as written. E.g. in the previous example, 1.20 g P 4
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