# Question #b69fa

Oct 7, 2014

in order to determine the mass of MgO produced when 50g of Mg react with ${\text{O}}_{2}$, you must use stoichiometry. This answer will give the theoretical yield of Mg. Your experimental yield will more than likely not be the same, as Mg can also react with oxygen in other ratios, and it can also react with water vapor, carbon dioxide, and nitrogen present in the atmosphere. Also, not all of the Mg may have reacted.

Step 1.
Write and balance the equation for the combustion of magnesium.
$\text{2Mg}$ + $\text{O"_2}$ $\rightarrow$ $\text{2MgO}$.

Step 2.
Convert what you have (50g Mg) to moles.
moles = grams/molar mass. Molar mass is the atomic weight in g/mol.

moles Mg = 50g Mg/ 24g/mol = 2.1 mol Mg

Step 3.
Using the coefficients in the balanced equation, convert moles of what you have (moles Mg) to moles of what you want (moles of MgO).

2.1 mol Mg x (2mol MgO/2 mol Mg) = 2.1 mol MgO

Step 4.
Convert moles from step 3 to grams.
moles x molar mass = grams

2.1 mol MgO x 40g/mol = 84g MgO = 80g MgO due to 50g Mg having only one significant figure.