Hướng dẫn giải

- PTHH:          4P  +  5${{O}_{2}}$ $\xrightarrow{{{t}^{0}}}$  2${{P}_{2}}{{O}_{5}}$    

                        ${{P}_{2}}{{O}_{5}}$ +  3${{H}_{2}}O$ $\to $  2${{H}_{3}}P{{O}_{4}}$

- ${{n}_{{{H}_{3}}P{{O}_{4}}}}=2{{n}_{{{P}_{2}}{{O}_{5}}}}=2.\frac{4}{2}{{n}_{P}}={{n}_{P}}=\frac{3,1}{31}=0,1$(mol) $\to {{C}_{M({{H}_{3}}P{{O}_{4}})}}=\frac{0,1}{0,1}=1M$

Chọn đáp án A.