Take the reaction: NH3 + O2 ---> NO + H2O. In an experiment, 3.25 g of NH3 are allowed to react with 3.50 g of O2. Which reactant is the limiting reagent?
You may either write the formula or the name of the limiting reactant.
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To determine which reactant is the limiting reagent, we need to compare the number of moles of each reactant and see which one is present in a lower amount.
First, we need to calculate the number of moles for each reactant using their respective masses and molar masses.
Molar mass of NH3 (molar mass of nitrogen + molar mass of hydrogen):
Molar mass of nitrogen (N) = 14.01 g/mol
Molar mass of hydrogen (H) = 1.01 g/mol
Molar mass of NH3 = (14.01 g/mol) + 3 * (1.01 g/mol) = 17.03 g/mol
Number of moles of NH3:
mol NH3 = mass NH3 / molar mass NH3
mol NH3 = 3.25 g / 17.03 g/mol ≈ 0.191 mol
Similarly, calculate the number of moles for O2:
Molar mass of O2 (2 * molar mass of oxygen):
Molar mass of oxygen (O) = 16.00 g/mol
Molar mass of O2 = 2 * (16.00 g/mol) = 32.00 g/mol
Number of moles of O2:
mol O2 = mass O2 / molar mass O2
mol O2 = 3.50 g / 32.00 g/mol ≈ 0.109 mol
Now that we have the number of moles for each reactant, we can compare them. The reactant with the lower number of moles is the limiting reagent. In this case, O2 has a smaller quantity of moles (0.109 mol) compared to NH3 (0.191 mol). Therefore, O2 is the limiting reagent in the reaction.
I do these the long way.
1. Balance the equation.
2. Convert g NH3 to mols. mols = grams/molar mass = ?
3. Convert g O2 to mols.
4. Using the coefficients in the balanced equation, convert mols NH3 5o mols NO.
5. Do the same and convert mols O2 to mols NO.
6. It is quite likely that these two values for mols NO produced will not be the same which means one of them is wrong. The correct value in limiting reagent problems is ALWAYS the smaller value.
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