Solution:
Step 1: Write the general formula.
[tex]\text{percent by mass of an element} = \frac{\text{mass of an element}}{\text{mass of a compound}} × 100[/tex]
Step 2: Calculate the percent composition of each element using the general formula.
For Mg
[tex]\text{percent by mass of Mg} = \frac{\text{mass of Mg}}{\text{mass of a sample}} × 100[/tex]
[tex]\text{percent by mass of Mg} = \frac{\text{0.82 g}}{\text{2.85 g}} × 100[/tex]
[tex]\boxed{\text{percent by mass of Mg = 28.77\%}}[/tex]
For C
[tex]\text{percent by mass of C} = \frac{\text{mass of C}}{\text{mass of a sample}} × 100[/tex]
[tex]\text{percent by mass of C} = \frac{\text{0.41 g}}{\text{2.85 g}} × 100[/tex]
[tex]\boxed{\text{percent by mass of C = 14.39\%}}[/tex]
For O
[tex]\text{percent by mass of O} = \frac{\text{mass of O}}{\text{mass of a sample}} × 100[/tex]
[tex]\text{percent by mass of O} = \frac{\text{1.62 g}}{\text{2.85 g}} × 100[/tex]
[tex]\boxed{\text{percent by mass of O = 56.84\%}}[/tex]
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