Chapter 9, Problem 66A

Interpretation Introduction

Interpretation : Whether the statement that the limiting reactant is always the reactant which is present in smallest amount with respect mass is true or false must be explained with the help of mathematical explanation.

Concept Introduction : Limiting reactant is the reactant which is consumed completely in a reaction.

Answer to Problem 66A

The limiting reactant is always the reactant which is present in smallest amount with respect mass. This is false statement.

Explanation of Solution

The statement is false as a limiting reactant might not be the reactant which is taken in less amount with respect to mass.

For example, 28 g nitrogen needs 6 g hydrogen gas for complete reaction. Now if 28 g nitrogen is mixed with 12 g hydrogen gas then 28 g nitrogen gas will be limiting reagent though the mass of hydrogen gas taken is 12 g which is lesser than that of nitrogen gas.

28 g nitrogen will react with 6 g hydrogen gas and there will be 6g hydrogen gas left.

This is because 1 mol nitrogen needs 3 moles hydrogen as shown below.

  ${\text{N}}_2{\text{(g)+3H}}_{\text{2}}\text{(g)}\to {\text{2NH}}_3\text{(g)}\text{.}$

Conclusion

Reason for false statements: The statement is false as limiting reactant is calculated based on the coefficients of the reactants in the balanced chemical equation and amount of reactants taken in moles.

Explanation :

  ${\text{N}}_2{\text{(g)+3H}}_{\text{2}}\text{(g)}\to {\text{2NH}}_3\text{(g)}\text{.}$

The reaction shows that moles of hydrogen required are more. But amount in g will be less for hydrogen gas as molecular weight is only 2 whereas the molecular weight of nitrogen is 28 g.

Conclusion: The given statement regarding limiting reactant is false.