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Propane $C_3H_8$ reacts with oxygen $O_2$ to form carbon dioxide $CO_2$ and water $H_2O$. Equation?
Reagents: $..C_3H_8$ $+$ $..O_2$ $\longrightarrow$ $...$
Products: $..C_3H_8$ $+$ $..O_2 \longrightarrow$ $..CO_2$ $+$ $..H_2O$
Let's take one molecule of propane: $\color{red}{1}C_3H_8$ $+$ $..O_2$ $\longrightarrow$ $..CO_2$ $+$ $..H_2O$
One molecule $C_3H_8$ contains 3 atoms $C$, one moleculee $CO_2$ contains only one. 3 molecules $CO_2$ will be formed: $1C_3H_8$ $+$ $..O_2$ $\longrightarrow$ $\color{red}{3}CO_2$ $+$ $..H_2O$
One molecule $C_3H_8$ contains 8 atoms $H$, one molecule $H_2O$ two $1C_3H_8$ $+$ $..O_2$ $\longrightarrow$ $3 CO_2$ $+$ $\color{red}{4}H_2O$
3 molecules $CO_2$ and 4 molecules $H_2O$ contain 10 atoms $O$, one molecule $O_2$ two. 5 molecules $O_2$ will be necessary: $1C_3H_8$ $+$ $\color{red}{5}O_2$ $\longrightarrow$ $3 CO_2$ $+$ $4 H_2O$
Balanced equation: $1C_3H_8$ $+$ $5O_2$ $\longrightarrow$ $3 CO_2$ $+$ $4 H_2O$