Question

how do you balance
C2H2+O2 → CO2+H2O

Answer 1

When balancing an equation you want to have the same number of each element on both sides of the reaction. Basically, the same number of Carbon, Hydrogen, and Oxygen on both sides.
\[2C _{2}H _{2}+5O _{2}\rightarrow4CO _{2}+2H _{2}O\]