Question

a 25.0 g sample of magnesium oxide (mgo) contains 10.8 g of magnesium. what is the percent of oxygen by mass in this compound?
a) 89.2%
b) 43.2%
c) 14.2%
d) 56.8%

Explanation:

Step1: Find mass of oxygen

The mass of the magnesium - oxide sample is $m_{total}=25.0$ g and the mass of magnesium in it is $m_{Mg}=10.8$ g. Using the formula $m_{O}=m_{total}-m_{Mg}$, we have $m_{O}=25.0 - 10.8=14.2$ g.

Step2: Calculate percent of oxygen by mass

The formula for percent by mass is $\text{Percent by mass}=\frac{m_{component}}{m_{total}}\times100\%$. Substituting $m_{O}=14.2$ g and $m_{total}=25.0$ g into the formula, we get $\frac{14.2}{25.0}\times100\% = 56.8\%$.

Answer:

D. 56.8%