Hướng dẫn giải

Áp dụng ĐLBTKL: m$\text{F}{{\text{e}}_{\text{2}}}{{\text{O}}_{\text{3}}}$+ m ${{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}$ = m_muối + ${{\text{m}}_{{{\text{H}}_{\text{2}}}\text{O}}}$

${{\text{m}}_{\text{muo }\!\!\acute{\mathrm{a}}\!\!\text{ i}}}$ = m$\text{F}{{\text{e}}_{\text{2}}}{{\text{O}}_{\text{3}}}$ + m${{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}$ -  m${{\text{H}}_{\text{2}}}\text{O}$ = 16 + 29,4 – 5,4 = 40 gam. Chọn đáp án C.