Which reagent is the limiting reagent in a reaction? A. The one with the largest molar mass
B. The one with the largest coefficient
C. The one with the smallest coefficient D. The one that runs out first in the reaction

D. The one that runs out first in the reaction

Explanation:

Limiting reagent is the one that limit the yield of product because of its lower amount and run out first. Lets take an example,

Mass of calcium carbonate = 25 g

Mass of hydrochloric acid = 13.0 g

Mass of calcium chloride produced = ?

Chemical equation:

CaCO₃ + 2HCl  → CaCl₂  + H₂O + CO₂

Number of moles of CaCO₃:

Number of moles of CaCO₃ = Mass /molar mass

Number of moles of CaCO₃= 25.0 g / 100.1 g/mol

Number of moles of CaCO₃ = 0.25 mol

Number of moles of HCl:

Number of moles of HCl = Mass /molar mass

Number of moles of HCl = 13.0 g / 36.5 g/mol

Number of moles of HCl = 0.36 mol

Now we will compare the moles of CaCl₂ with HCl and CaCO₃ .

                  CaCO₃      :               CaCl₂

                  1               :               1

                 0.25           :            0.25

                HCl              :                CaCl₂

                 2                :                 1

                 0.36            :               1/2 × 0.36 = 0.18 mol

The number of moles of CaCl₂ produced by HCl are less it will be limiting reactant.

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