Which of the following statements is false ?
Show Hint
- \(\mathrm{CrO_4^{2-}}\) is tetrahedral in shape
- \(\mathrm{Cr_2O_7^{2-}}\) has a Cr-O-Cr bond
- \(\mathrm{Na_2Cr_2O_7}\) is a primary standard in volumetry
- \(\mathrm{Na_2Cr_2O_7}\) is less soluble than \(\mathrm{K_2Cr_2O_7}\)
The Correct Option is C
Solution and Explanation
(2) \(\mathrm{Cr_2O_7^{2-}}\) consists of two \(\mathrm{CrO_4}\) tetrahedra sharing a corner oxygen, i.e. a Cr-O-Cr bridge: true.
(3) A primary standard must be pure, stable and non-hygroscopic. \(\mathrm{Na_2Cr_2O_7}\) is hygroscopic (it is obtained as \(\mathrm{Na_2Cr_2O_7\cdot 2H_2O}\)) and cannot be weighed accurately, whereas \(\mathrm{K_2Cr_2O_7}\) is the primary standard. So (3) is false.
(4) Note that \(\mathrm{Na_2Cr_2O_7}\) is in fact more soluble than \(\mathrm{K_2Cr_2O_7}\) (which is why the potassium salt is crystallised from sodium dichromate with KCl), so (4) as printed is also incorrect, but (3) is the intended false statement.
Answer: (3) \(\mathrm{Na_2Cr_2O_7}\) is a primary standard in volumetry
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