# Question f349d

May 12, 2015

You use the mole ratio that exists between $\text{A}$ and $\text{B}$.

Let's say your balanced equation looks like this

$\text{A" + color(red)(2)"B" -> "products}$

If you have 1 mole of $\text{A}$, you'd need $\textcolor{red}{2}$ moles of $\text{B}$ for the reaction to take place. This means that you have a $1 : 2$ mole ratio between $\text{A}$ and $\text{B}$.

So, if you have $x$ moles of $\text{A}$, you'd have

xcancel("molesA") * (color(red)(2)" moles B")/(1cancel("mole A")) = "2x moles B"#

So, if you know the number of moles of a species involved in the reaction, you can automatically use the stoichiometric coefficients to determine the number of moles of the rest of the species present.

If you have a balanced chemical equation that looks like this

$\textcolor{red}{2} \text{A" + C -> color(blue)(3)"B} + D$

You'd have a $\textcolor{red}{2} : \textcolor{b l u e}{3}$ mole ratio between $\text{A}$ and $\text{B}$, so you can go from moles of $\text{A}$ to moles of $\text{B}$ by multiplying by 3/2

$\text{x"cancel("moles A") * (color(blue)(3)" moles B")/(color(red)(2)cancel("moles A")) = 3/2"x moles B}$