\(\rightarrow\left\{{}\begin{matrix}Na^+:1,75\\Cl^-:1,5\\\rightarrow AlO_2^-:0,25\end{matrix}\right.\)
\(\Rightarrow n_{Al^{3+}}=0,25\left(mol\right)\)
\(n_{OH^-}=1,75\left(mol\right)\)
\(\Rightarrow n_{Al\left(OH\right)_3}=n_{Al^{3+}}=0,25\left(mol\right)\)
\(\Rightarrow m=78.0,25=19,5\left(g\right)\)