The number of white coloured salts among the following is ___________.
(A) SrSO\(_4\), (B) Mg(NH\(_4\))PO\(_4\), (C) BaCrO\(_4\), (D) Mn(OH)\(_2\), (E) PbSO\(_4\), (F) PbCrO\(_4\), (G) AgBr, (H) PbI\(_2\), (I) CaC\(_2\)O\(_4\),(J) [Fe(OH)\(_2\)(CH\(_3\)COO)\(_2\)]
The number of white coloured salts among the following is ___________.
(A) SrSO\(_4\), (B) Mg(NH\(_4\))PO\(_4\), (C) BaCrO\(_4\), (D) Mn(OH)\(_2\), (E) PbSO\(_4\), (F) PbCrO\(_4\), (G) AgBr, (H) PbI\(_2\), (I) CaC\(_2\)O\(_4\),(J) [Fe(OH)\(_2\)(CH\(_3\)COO)\(_2\)]
Correct Answer: 5
Solution and Explanation
The problem asks to count the number of white colored salts from a given list of compounds. The color of an ionic compound is typically determined by the color of its constituent ions. Cations of s-block and p-block elements are generally colorless, leading to white salts unless the anion is colored. Cations of d-block (transition metals) are often colored due to d-d electronic transitions. Certain anions like chromate (CrO₄²⁻), dichromate (Cr₂O₇²⁻), and permanganate (MnO₄⁻) are also colored.
Step-by-Step Solution:
We will analyze the color of each salt individually.
(A) SrSO₄ (Strontium Sulfate):
The cation is Sr²⁺ (Strontium, an s-block element) and the anion is SO₄²⁻ (sulfate). Both ions are colorless. Therefore, SrSO₄ is a white solid.
(B) Mg(NH₄)PO₄ (Magnesium Ammonium Phosphate):
The cations are Mg²⁺ (s-block) and NH₄⁺ (ammonium), and the anion is PO₄³⁻ (phosphate). All three ions are colorless. Therefore, Mg(NH₄)PO₄ is a white solid.
(C) BaCrO₄ (Barium Chromate):
The cation is Ba²⁺ (s-block), which is colorless. However, the anion is CrO₄²⁻ (chromate), which is distinctly yellow. Therefore, BaCrO₄ is a yellow salt, not white.
(D) Mn(OH)₂ (Manganese(II) Hydroxide):
The cation is Mn²⁺ (a d-block transition metal ion). The Mn²⁺ ion has a d⁵ electron configuration. The d-d transitions are spin-forbidden, so its compounds are very pale in color. Freshly precipitated Mn(OH)₂ is a white solid, although it quickly oxidizes in air and turns brown. In its pure state, it is considered white.
(E) PbSO₄ (Lead(II) Sulfate):
The cation is Pb²⁺ (a p-block element) and the anion is SO₄²⁻. Both ions are colorless. Therefore, PbSO₄ is a white solid.
(F) PbCrO₄ (Lead(II) Chromate):
The cation is Pb²⁺, which is colorless. The anion is the yellow chromate ion, CrO₄²⁻. Therefore, PbCrO₄ is a bright yellow solid, not white.
(G) AgBr (Silver Bromide):
The cation Ag⁺ and anion Br⁻ are both colorless. However, due to polarization and charge-transfer effects, AgBr is not pure white but rather a pale yellow or cream-colored solid. It is not considered white.
(H) PbI₂ (Lead(II) Iodide):
The cation Pb²⁺ is colorless. The anion is I⁻ (iodide). Due to the high polarizability of the iodide ion, PbI₂ is a bright yellow solid, not white.
(I) CaC₂O₄ (Calcium Oxalate):
The cation is Ca²⁺ (s-block) and the anion is C₂O₄²⁻ (oxalate). Both ions are colorless. Therefore, CaC₂O₄ is a white solid.
(J) [Fe(OH)₂(CH₃COO)₂] (Basic Iron(II) Acetate):
This compound contains the Fe²⁺ (Iron(II)) ion, which is a transition metal ion. Fe²⁺ compounds are typically colored, often appearing pale green or greenish-white. They are not pure white.
Final Computation & Result:
Let's list the salts identified as white:
- (A) SrSO₄
- (B) Mg(NH₄)PO₄
- (D) Mn(OH)₂
- (E) PbSO₄
- (I) CaC₂O₄
Counting these salts, we find that there are 5 white colored salts in the given list.
The number of white coloured salts is 5.
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