Write a balanced half-reaction for the reduction of gaseous oxygen to aqueous hydrogen peroxide in acidic aqueous solution. be sure to add physical state symbols where appropriate.

First, the reduction  of gaseous oxygen to aqueous hydrogen peroxide is:

O2 (g) → H2O2 (l)

when this reaction in acidic aqueous solution, and we need to balance H on both sides so we will use H+ on the left side.

O2(g) + H+(aq) → H2O2(l)

now we have 1 of H atom on the left side and 2 in the right side so we will but 2H+ on the left side instead of H+:

O2(g) + 2H+(aq)  → H2O2(l)

now we need to balance the 2 positive charge of 2H+ so, we will put 2 e- on the left side:

O2(g) + 2H+(aq) + 2 e- → H2O2(l)

and this is the final balanced equation which has equal no. of atoms on both sides:

2 of O atom, 2 of H atom on both sides.