I'm given the following problem:
Write the molecular equation for the reaction $\mathrm{Iron\;(III)\; Nitrate\; and\; Sodium \;Phosphate}$
I begin by attempting to determine the empirical formula of $\mathrm{Iron\;(III)\; Nitrate}$. In class, we were taught to memorize the following table:
Group Charge
#1 +1
#2 +2
#3 +3
#5 -3
#6 -2
#7 -1
Additionally, we were asked to memorize the "formula" for Nitrate ($\mathrm{NO_3}$).
The $\mathrm{III}$ after Iron indicates that it has a $3+$ charge. From the table above, $\mathrm{N}$ has charge $3-$, and $\mathrm{O}$ has charge $2-$. There are $3$ $\mathrm{O's}$, giving a net charge of $-2 \cdot 3=-6$.
Unable to determine the empirical formula of $\mathrm{Iron\;(III)\; Nitrate}$, I looked it up to find $\mathrm{Fe(NO_3)_3}$.
If $\mathrm{NO_3}$ has charge $6-$, then $3$ $\mathrm{NO_3}$'s has charge $18-$. Meanwhile, $\mathrm{Fe}$ has only charge $3+$.
As it turns out, $NO_3$ actually has charge $1-$. How is this determined? I'm told its "just something that needs to be memorized" in order to progress in the course.