Chapter 9, Problem 8ALQ

Interpretation Introduction

Interpretation:

For the given equation, if $4$ moles of A is reacted with $2$ moles of B, then the correct statement is to be predicted. The reason behind the incorrect statements is to be stated.

Concept Introduction:

Limiting reagent is the reactant in the chemical reaction which is completely consumed. The amount of product and the excess reactant left in a reaction is determined on the basis of limiting reagent only.

Answer to Problem 8ALQ

The true statement is “A is the limiting reactant because you need $6$ moles of A and have 4 moles”.

Explanation of Solution

Reason for true statements: Possibility 1 is correct as in possibility 2 there are negative numbers of moles of A which is not possible.

The mole ratio for A to C or D is $3:1$, that is for three moles of A one mole of both C and D are formed. The mole ratio for B to C or D is $1:1$, that is for one mole of B one mole of both C and D are formed.

There are two possibilities for the limiting reagent in the given reaction.

Possibility 1

$\begin{array}{l}3\text{A}+\text{B}\to \text{C}+\text{D}\\ \text{Before 4 }\text{2 }\text{0 }\text{0}\\ \text{Change }-4-1.34+1.34+1.34\\ \text{After 0 0}\text{.66 }1.341.34\end{array}$

In this case, A is considered as limiting reagent.

Possibility 2

$\begin{array}{l}3\text{A}+\text{B}\to \text{C}+\text{D}\\ \text{Before 4 }\text{2 }\text{0 }\text{0}\\ \text{Change }-6-2+2+2\\ \text{After }-2\text{ 0 }22\end{array}$

In this case, B is considered as limiting reagent.

Possibility 1 is correct as in possibility 2 there are negative numbers of moles of A which is not possible.

Conclusion

Reason for false statements:

(a) The given statement is not true as the molar mass is not required for determining the limiting reactant.

(c) The given statement is not true because possibility 1 is correct. In possibility 2, there are negative numbers of moles of A which is not possible.

(d) The given statement is not true because possibility 1 is correct. In possibility 2, there are negative numbers of moles of A which is not possible.

(e) The given statement is not true because A acts as limiting reagent.

Explanation:

(a) It is not necessarily true that the limiting reactant is the one with higher molar mass. The limiting reactant is calculated on the basis of stoichiometry of reactants in a chemical reaction.

(c) The mole ratio for A to C or D is $3:1$, that is for three moles of A one mole of both C and D are formed. The mole ratio for B to C or D is $1:1$, that is for one mole of B one mole of both C and D are formed.

There are two possibilities for the limiting reagent in the given reaction.

Possibility 1

$\begin{array}{l}3\text{A}+\text{B}\to \text{C}+\text{D}\\ \text{Before 4 }\text{2 }\text{0 }\text{0}\\ \text{Change }-4-1.34+1.34+1.34\\ \text{After 0 0}\text{.66 }1.341.34\end{array}$

In this case, A is considered as limiting reagent.

Possibility 2

$\begin{array}{l}3\text{A}+\text{B}\to \text{C}+\text{D}\\ \text{Before 4 }\text{2 }\text{0 }\text{0}\\ \text{Change }-6-2+2+2\\ \text{After }-2\text{ 0 }22\end{array}$

In this case, B is considered as limiting reagent.

Possibility 1 is correct as in possibility 2 there are negative numbers of moles of A which is not possible.

(d) The mole ratio for A to C or D is $3:1$, that is for three moles of A one mole of both C and D are formed. The mole ratio for B to C or D is $1:1$, that is for one mole of B one mole of both C and D are formed.

There are two possibilities for the limiting reagent in the given reaction.

Possibility 1

$\begin{array}{l}3\text{A}+\text{B}\to \text{C}+\text{D}\\ \text{Before 4 }\text{2 }\text{0 }\text{0}\\ \text{Change }-4-1.34+1.34+1.34\\ \text{After 0 0}\text{.66 }1.341.34\end{array}$

In this case, A is considered as limiting reagent.

Possibility 2

$\begin{array}{l}3\text{A}+\text{B}\to \text{C}+\text{D}\\ \text{Before 4 }\text{2 }\text{0 }\text{0}\\ \text{Change }-6-2+2+2\\ \text{After }-2\text{ 0 }22\end{array}$

In this case, B is considered as limiting reagent.

Possibility 1 is correct as in possibility 2 there are negative numbers of moles of A which is not possible.

(e) The mole ratio for A to C or D is $3:1$, that is for three moles of A one mole of both C and D are formed. The mole ratio for B to C or D is $1:1$, that is for one mole of B one mole of both C and D are formed.

There are two possibilities for the limiting reagent in the given reaction.

Possibility 1

$\begin{array}{l}3\text{A}+\text{B}\to \text{C}+\text{D}\\ \text{Before 4 }\text{2 }\text{0 }\text{0}\\ \text{Change }-4-1.34+1.34+1.34\\ \text{After 0 0}\text{.66 }1.341.34\end{array}$

In this case, A is considered as limiting reagent.

Possibility 2

$\begin{array}{l}3\text{A}+\text{B}\to \text{C}+\text{D}\\ \text{Before 4 }\text{2 }\text{0 }\text{0}\\ \text{Change }-6-2+2+2\\ \text{After }-2\text{ 0 }22\end{array}$

In this case, B is considered as limiting reagent.

Possibility 1 is correct as in possibility 2 there are negative numbers of moles of A which is not possible.

Possibility 1 is correct as in possibility 2 there are negative numbers of moles of A which is not possible.