Question

For the reaction ${\text{2P}}\,{\text{ + }}\,{\text{Q}}\,\, \to \,{\text{R}}\,$, $8$ mol of P and $5$ mol of Q will produce:
A. $8$mol of R
B. $5$mol of R
C. $4$mol of R
D. $13$mol of R

Answer 1

To answer this question we should know what is limiting reagent and how to find it. By comparing the mole ratio of both reactants according to the given equation we will determine the limiting reagent. After finding the limiting reagent, by comparing the amount of limiting reagent with the product according to the given reaction, we will determine the amount of product.

Complete answer:
The reactant which is present in a low amount in the reaction is known as the limiting reagent.
We can find the limiting reagent as follows:
The given reaction is as follows:
${\text{2P}}\,{\text{ + }}\,{\text{Q}}\,\, \to \,{\text{R}}\,$
According to the above reaction, two moles of P is reacting with one mole of Q so, eight moles of P will react,
$2$ mol of P = $1$ mol of Q (according to the given reaction)
$8$ mol of P = $4$ mol of Q
So, eight moles of P will react with four moles of Q and we have five moles of Q means Q is in excess.

Similarly, According to the above reaction, one mole of Q is reacting with two moles of P so, five moles of Q will react,
$1$ mol of Q = $2$ mol of P (according to the given reaction)
$5$ mol of Q = $10$ mol of P
So, five moles of Q will react with ten moles of P and we have only eight moles of P means P is in low amounts. So, we can say as P is less amount means P is limiting reagent. So, the amount of product will be decided on the basis of the amount of limiting reagent P.

Now we will compare the amount of limiting reagent with the given equation to determine the amount of product R as follows:
$2$ mol of P = $1$ mol of R (according to the given reaction)
$8$ mol of P = $4$ mol of R
So, $8$ mol of P and $5$ mol of Q will produce $4$ mol of R.

Therefore, option (C) $4$ mol of R, is the correct answer.

Note: Limiting reagent decides the amount of product because all reactants react in fixed composition as the limiting reagent is in less amount, so it will consume first and other reactant remains in excess. This comparison of the coefficient is known as the stoichiometric comparison.
For a stoichiometric comparison, a balanced equation is necessary. If the compounds or molecules are given in place of alphabets it is necessary to balance the given equation first. When the reactants are reacting in a one-one ratio then the reactant which is given in a low amount will be the limiting reagent.
When the stoichiometry of reactants differs then we have to determine the limiting reagent because in that case, the one which is given in high amounts can also be the limiting reagent it depends upon the stoichiometry.