$FeCr _{2} O _{4}+ Na _{2} CO _{3}+ O _{2} \rightarrow A + Fe _{2} O _{3}+ CO _{2}$
$A + H ^{+} \rightarrow B + H _{2} O + Na ^{+}$
\(+8 CO _{2}\)
\(2 Na _{2} CrO _{4}+2 H ^{+} \rightarrow \underbrace{ Na _{2} Cr _{2} O _{7}}_{ B }+2 Na ^{+}+ H _{2} O\)