Question

In a reaction
$A+B_2\to A{B}_2$

Identify the limiting reagent, if any, in the following reaction mixtures

$A$: $300$ atoms of $A$ +$200$ molecules of $B$

$B$: $2$ mol $A$+$3$ mol $B$

$C$: $100$ atoms of $A$ +$100$ molecules of $B$

$D$: $5$ mol $A$ +$2.5$ mol $B$

$E$: $2.5$ mol$A$+$5$ mol $B$

Answer 1

$A$:

$300\text{atoms of}A+200\text{molecules of}B$

$1\text{atom of}A\text{reacts with}1\text{molecule of}B$. Similarly, $200\text{atoms of}A\text{react with}200\text{molecules of}B$, so $100$ atoms of A are unused.
Hence, $B$ is the limiting reagent.


$B$:

$2\text{mol of}A+3\text{mol of}B$

$1\text{mole of}A\text{reacts with}1\text{mole of}B$. Similarly, $2\text{moles of}A\text{reacts with}2\text{moles of}B$, so $1\text{mole of}B$ is unused.
Hence, $A$ is the limiting reagent.

$C$:
$100\text{atoms of}A+100\text{molecules of}B$

$1\text{atom of}A\text{reacts with}1\text{molecule of}Y$. Similarly, $100\text{atoms of}A\text{react with}100\text{molecules of}B$.
Hence, it is a stoichiometric mixture where there is no limiting reagent.


$D$:

$5\text{mole}A+2.5\text{mol}B$

$1\text{mole of}A\text{reacts with}1\text{mole of}B$. Similarly, $2.5\text{moles of}A\text{reacts with}2.5\text{moles of}B$, so $2.5\text{moles of}A$ is unused.
Hence, $B$ is the limiting reagent.

$E$:

$2.5\text{mol}A+5\text{mol}B$

$1\text{mole of}A\text{reacts with}1\text{mole of}B$. Similarly, $2.5\text{moles of}A\text{reacts with}2.5\text{moles of}B$, so $2.5\text{moles of}B$ is unused.