a reaction vessel contains a small piece of magnesium metal and air (20%) of which is oxygen gas. The mixture is ignited and burns with a burst of light and heat, producing solid magneisum oxide. The mass of mgO, however, is less than the initital mass of magnesium and oxygen combined. What loss of mass

see below

Explanation:

Mg burning in air gives MgO and a smaller amount of Mg3N2

2Mg+O2=2MgO. 2*24.3+32=2*40.3

3Mg+N2=Mg3N2

Ignoring the nitride, MgO loss of mass during the reaction can be ascribed to

(1) relativistic loss as energy (too small to be measurable)

(2) During the reaction, which is strongly exothermic, a fraction f the material is blown off as tiny particles which adhere to the vessel.