You know that you will need a balanced chemical equation with masses, moles, and molar masses, etc.
Step 1. Assemble all the information in one place
M_text(r):color(white)(mmm)302.35color(white)(mmmmmmmll)32.00Mr:mmm302.35mmmmmmmll32.00
color(white)(mmmm)"2Cr"("ClO"_3)_3 → "2CrCl"_3 + "9O"_2mmmm2Cr(ClO3)3→2CrCl3+9O2
m"/g": color(white)(mmll)146.5m/g:mmll146.5
Step 2. Calculate the moles of "Cr"("ClO"_3)_3Cr(ClO3)3
"Moles of Cr"("ClO"_3)_3 = 146.5 color(red)(cancel(color(black)("g Cr"("ClO"_3)_3))) × ("1 mol Cr"("ClO"_3)_3)/(302.35 color(red)(cancel(color(black)("g Cr"("ClO"_3)_3)))) = 0.4845 color(white)(l) "mol Cr"("ClO"_3)_3
Step 3 Calculate the moles of "O"_2
"Moles of O"_2 = 0.4845 color(red)(cancel(color(black)("mol Cr"("ClO"_3)_3)))× ("9 mol O"_2)/(2 color(red)(cancel(color(black)("mol Cr"("ClO"_3)_3)))) = "2.180 mol O"_2
Step 3. Calculate the mass of "O"_2
"Mass of O"_2 = 2.180 color(red)(cancel(color(black)("mol O"_2))) × "32.00 g O"_2/(1 color(red)(cancel(color(black)("mol O"_2)))) = "69.77 g O"_2