# Question #759d0

Nov 9, 2015

$M g O \left(s\right)$ + $2 H C l \left(a q\right)$ $\rightarrow$ $M g C {l}_{2} \left(s\right)$ + ${H}_{2} O \left(l\right)$

#### Explanation:

To convert chemical names into formulas/equations, one needs to be familiar with the names and chemical symbols of ions.

In naming ionic compounds, the metal cation (positive charge) goes first followed by the nonmetal anion's (negative charge) name. So for magnesium oxide:

Cation: magnesium
Anion: oxide

The image below depicts the monoatomic ions with their respective charges. Notice that for magnesium the charge is +2 while for oxygen (or oxide) it is -2. Thus,

Cation: magnesium = $M {g}^{\text{2+}}$
Anion: oxide = ${O}^{\text{2-}}$

So converting the name magnesium oxide, we have

$\textcolor{red}{M {g}_{2} {O}_{2}}$ (incorrect)

but since the subscripts are both divisible by the factor, 2 then we can further reduce the chemical formula to its correct form,

$\textcolor{b l u e}{M g O}$ (correct)

Now let's try it with magnesium chloride.

Cation: magnesium = $M {g}^{\text{2+}}$
Anion: chloride = $C {l}^{\text{1-}}$

converting the name into chemical formula we have,

$\textcolor{red}{M {g}_{1} C {l}_{2}}$ but since the subscript of $M g$ is one, we can rewrite the formula to its proper form, $\textcolor{b l u e}{M g C {l}_{2}}$ instead.

Notice that for the subscripts, we have just been 'exchanging the superscripts of the ions to become the subscript of its partner ion'.

Naming simple binary acids (always composed of one hydrogen atom and one monoatomic anion), on the other hand, follows these steps: prefix $\textcolor{\mathmr{and} a n \ge}{h y \mathrm{dr} o -}$ + $\text{first syllable of the anion}$ + suffix $\textcolor{m a \ge n t a}{\text{-ic acid}}$.

So for the substance $\textcolor{\mathmr{and} a n \ge}{h y \mathrm{dr} o} \text{chlor} \textcolor{m a \ge n t a}{i c}$ $\textcolor{m a \ge n t a}{a c i d}$, we now know that

Cation: hydrogen = ${H}^{\text{1+}}$
Anion: chloride = $C {l}^{\text{1-}}$

Therefore the proper chemical formula is $\textcolor{b l u e}{H C l}$.

Thus, we can translate the word equation into the chemical one:

$M g O \left(s\right)$ + $H C l \left(a q\right)$ $\rightarrow$ $M g C {l}_{2} \left(s\right)$ + ${H}_{2} O \left(l\right)$ (unbalanced)

to balance the equation, one just needs to tally the atoms based on the subscripts.

$M g O \left(s\right)$ + $\textcolor{g r e e n}{2} H C l \left(a q\right)$ $\rightarrow$ $M g C {l}_{2} \left(s\right)$ + ${H}_{2} O \left(l\right)$ (unbalanced)

left side: Mg = 1 ; O= 1; H = (1 x $\textcolor{g r e e n}{2}$) = 2; Cl = (1 x $\textcolor{g r e e n}{2}$) = 2
right side: Mg = 1 ; O = 1 ; H = 2; Cl = 2

The equation is now balance.