MCQ
Which of the following statements is false ?
- ✓$Na_2Cr_2O_7$ is less soluble than $K_2Cr_2O_7$
- B$Na_2Cr_2O_7$ is primary standard in volumetry
- C$CrO_4^{2-}$ is tetrahedral in shape
- D$CrO_7^{2-}$ has a $Cr- O- Cr$ bond
Hydration energy of $Na^+$ is greater then $K^+$. Because of smaller size of $Na^+$ , $Na_2Cr_2O_7$ is more soluble than $K_2Cr_2O_7$. 
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Compound $(D)$ is sodium hexametaphosphate which is known as
$IO_3^-+ aI^-+ bH^+ \to cH_2O + dI_2$
$a, b, c, d$ respectively corresponds to