31 gram fosfor dibakar dan membentuk oksida fosfor, kemudian oksida fosfor tersebut dilarutkan dalam air sehingga terbentuk asam fosfat.
Jumlah mol P mula-mula
\begin{equation*}
\begin{split}
\text{n} & = \frac {\text{m}}{\text{Ar}} \\\\
\text{n} & = \frac {31}{4 \:.\: 31} \\\\
\text{n} & = \frac 14 \text{ mol}
\end{split}
\end{equation*}
Reaksi 1
\begin{equation*}
\begin{array}
& \ce{P4 (s) & + & 5 O2 (g) & -> & 2 P2O5 (g) } \\\\
\dfrac 14 \text{ mol} && \dfrac 54 \text{ mol} && \color {red} {\dfrac 12 \text{ mol}}
\end{array}
\end{equation*}
Reaksi 2
\begin{equation*}
\begin{array}
& \ce{P2O5 (g) & + & 3 H2O (l) & -> & 2 H3PO4 (aq)} \\\\
\color {red} {\dfrac 12 \text{ mol}} && \dfrac 32 \text{ mol} && \color {blue} {1 \text{ mol}}
\end{array}
\end{equation*}
Massa \(\ce{H3PO4}\) yang terbentuk
\begin{equation*}
\begin{split}
\text{n} & = \frac {\text{m}}{\text{Mr}} \\\\
1 & = \frac {\text{m}}{98} \\\\
\text{m} & = 392 \text{ gram}
\end{split}
\end{equation*}
