JEE Main 2015 — d and f Block Elements Question with Solution

It is true ${\text{Na}}_{\text{2}}{\text{Cr}}_{\text{2}}{\text{O}}_{\text{7}}$ is more soluble than ${\text{K}}_{\text{2}}{\text{Cr}}_{\text{2}}{\text{O}}_{\text{7}}$.
${\text{CrO}}_4^{-2}$ Is tetrahedral:
 

$\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7$ is not the primary standard but ${\mathrm{K}}_2\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7$ (acidic) is the primary standard reagent for titration because sodium dichromate is Hygroscopic in nature, that means it absorb moisture from atmosphere, that will cause incorrect reading in titration.