What is the percent composition by mass of oxygen in magnesium oxide? Chemistry The Mole Concept Percent Composition 1 Answer anor277 Nov 5, 2016 #"Percentage oxygen composition in magnesium oxide"~=40%#. Explanation: #"Percentage oxygen composition in magnesium oxide "=# #"Mass of oxygen"/"Mass of oxygen + mass of magnesium"xx100%# #=# #(15.999*cancel(g*mol^-1))/(15.999*cancel(g*mol^-1)+24.305*cancel(g*mol^-1))xx100%=??# Answer link Related questions What is mole percent? Question #e2e8b Question #bc737 Question #bc73b Question #e2e8f Question #b9857 Question #b985b Question #fee41 Question #c5c01 Question #bc73f See all questions in Percent Composition Impact of this question 23715 views around the world You can reuse this answer Creative Commons License