Answer
a) $4Cr + 3O_{2} → 2Cr_{2}O_{3}$
b) $Cu_{2}S + O_{2} → 2Cu + SO_{2}$
c) $C_{6}H_{5}CH_{3} + 9O_{2} → 4H_{2}O + 7CO_{2}$
Work Step by Step
a) $4Cr + 3O_{2} → 2Cr_{2}O_{3}$
four moles of chromium reacts with three moles of oxygen to form two moles of chromium oxide.
b) $Cu_{2}S + O_{2} → 2Cu + SO_{2}$
one mole of copper sulfide reacts with one mole of oxygen to form two moles of copper in free state and one mole of sulfer dioxide.
c) $C_{6}H_{5}CH_{3} + 9O_{2} → 4H_{2}O + 7CO_{2}$
one mole of toluene reacts with nine moles of oxygen to form four moles of water and seven moles of carbon dioxide.
