Question

% element = (mass of element in formula / molar mass of compound) × 100

1. calculate the percent composition of each element in mgo.
2. determine the percent composition of each element in alcl₃.

Explanation:

Step1: Find molar masses of elements

Atomic mass: Mg=24.31, O=16.00, Al=26.98, Cl=35.45

Step2: Calculate MgO molar mass

Molar mass of MgO = 24.31 + 16.00 = 40.31

Step3: Compute %Mg and %O in MgO

%Mg = $\frac{24.31}{40.31}×100 ≈ 60.3\%$
%O = $\frac{16.00}{40.31}×100 ≈ 39.7\%$

Step4: Calculate AlCl₃ molar mass (corrected formula)

Molar mass of AlCl₃ = 26.98 + 3×35.45 = 26.98 + 106.35 = 133.33

Step5: Compute %Al and %Cl in AlCl₃

%Al = $\frac{26.98}{133.33}×100 ≈ 20.2\%$
%Cl = $\frac{106.35}{133.33}×100 ≈ 79.8\%$

Answer:

1. %Mg ≈ 60.3%, %O ≈ 39.7%
2. %Al ≈ 20.2%, %Cl ≈ 79.8% (assuming AlCl₃)