Yellow precipitate in the presence of ammonium molybdate?
[...] diluted $\ce{H2SO4}$ and $\ce{HNO3}$ [...] $\ce{NH4OH}$ [...] and ammonium molybdate.


The yellow precipitate indicates that phosphate was present too. Under these conditions a yellow ammonium molybdatophosphate is formed:

$\ce{H2PO4^- + 22 H+ + 12 MoO4^2- +3 NH4+ -&gt; \underset{yellow}{(NH4)3[P(Mo3O10)4]} +12H2O}$