# How do I convert grams to moles to mole ratios? "Mg" + "O"_2 -> "MgO" 16.2g+10.67g=26.86g

Mar 22, 2017

Here's how you can do that.

#### Explanation:

For starters, make sure that the chemical equation is balanced

$\textcolor{b l u e}{2} {\text{Mg"_ ((s)) + "O"_ (2(g)) -> color(purple)(2)"MgO}}_{\left(s\right)}$

Notice that the reaction consumes $\textcolor{b l u e}{2}$ moles of magnesium and $1$ mole of oxygen gas and produces $\textcolor{p u r p \le}{2}$ moles of magnesium oxide.

You can thus say that the number of moles of each chemical species that takes part in this reaction must satisfy

$\text{moles Mg " : " moles O"_2color(white)(.): " moles MgO}$

color(white)(aaaacolor(blue)(2)aaaa. color(black)(":") aaaacolor(black)(1) aaaaacolor(black)(":") aaaaacolor(purple)(2)

Now, you can convert grams to moles by using molar mass. In this case, you have

M_ ("M Mg") = "24.3050 g mol"^(-1)

M_ ("M O"_2) = "32.0 g mol"^(-1)

M_ ("MgO") = "40.3050 g mol"^(-1)

So, to convert the mass of magnesium from grams to simply, use the element's molar mass as a conversion factor

16.2 color(red)(cancel(color(black)("g"))) * "1 mole Mg"/(24.3050 color(red)(cancel(color(black)("g")))) = "0.6665 moles Mg"

Do the same for the sample of oxygen gas.

10.67 color(red)(cancel(color(black)("g"))) * "1 mole O"_2/(32.0color(red)(cancel(color(black)("g")))) = "0.3334 moles O"_2

Finally, do the same for the magnesium oxide.

26.86 color(red)(cancel(color(black)("g"))) * "1 mole MgO"/(40.3050color(red)(cancel(color(black)("g")))) = "0.6664 moles MgO"

So, you know that when $0.6665$ moles of magnesium react with $0.3334$ moles of oxygen gas, you get $0.6664$ moles of magnesium oxide.

The values are not an exact match because of rounding and of the values used for the molar masses of the three chemical species.