How to Solve Limiting/Excess Reactant Problems
WS9.8
In nature and in the lab, substance are being mixed together in all kinds of amounts. How much product will form? Here is a typical example. Example: If I mix together 10 grams of hydrogen with 10 grams of nitrogen, how many grams of ammonia (NH3) will I make? First we need to write a balanced chemical reaction: 3 H2 + N2 2 NH3 Next, we see how much product each reactant can make. The one that makes less is the limiting reactant. It’s like it sounds- it limits how much product can be made.
mole N2 2 mole NH3 17 g NH3 x x 12.1 g NH3 28 g N2 mole N2 mole NH3 mole H2 2 mole NH3 17 g NH3 10 g H2 x x x 56.7 g NH3 2 g H2 3 mole H2 mole NH3 10 g N2 x
This tells us that nitrogen is the limiting reactant. We can see how much H2 was wasted by seeing how much is needed:
10 g N2 x
2 g H2 mole N2 3 mole H2 x x 2.1 g H2 28 g N2 mole N2 mole H2
The rest of the hydrogen (7.9 g) is wasted. In summary, to perform limiting reactant problems 1. Write a balanced chemical reaction 2. Identify the limiting reactant by calculating how much product each reactant can make. The one that makes less is the limiting reactant. 3. To figure out how much excess reactant there is, see how much is needed and compare that to what you used. Use this example to guide you through these 2 problems. 1. Determine the mass of Tetraphosphorus Decoxide (P4O10) which is formed from 25.0 grams of Phosphorus (P4) and 50.0 grams of Oxygen gas (O2).
2. You start with 200.0 grams of Vanadium (V) and 100.0 grams of Oxygen Gas (O2) and the only product is Vanadium Oxide (V2O5). a) What is the limiting reactant? b) How many grams of Vanadium Oxide (V2O5) can form? c) How much excess reactant remains?
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