H 3 PO 4 is a tribasic acid with pK _ a _ 1 , pK _ a _ 2 and pK _ a _ 3 2.12, 7.21, and…

Chemistry · JEE Advanced · NTA ExamsEquilibrium


H3PO4 is a tribasic acid with \(\mathrm{pK}_{\mathrm{a}_{1}}, \mathrm{pK}_{\mathrm{a}_{2}}\) and \(\mathrm{pK}_{\mathrm{a}_{3}}\) 2.12, 7.21, and 12.32, respectively. It is used in fertiliser productions and its various salts are used in food, detergent, toothpaste, and in metal treatment.
Small quantities of H3PO4 are used in imparting the sour or tart taste to soft drinkes, such as Coca Cola, and beers, in which H3PO4 in present 0.05% by weight (density = 1.0 g mL–1).
10–3 M H3PO4 (pH = 7) is used in fertilisers as an aqueous soil digesting. Plants can absorb zinc in water soluble form only. Zinc phosphate is the source of zinc and \(\mathrm{PO}_{4}^{3-}\) ions in the soil. Ksp of zinc phosphate
= 9.1 × 10–33.
 
\(\left[\mathrm{PO}_{4}^{3-}\right]\) ion in the soil with pH = 7, is
  1. 10–3 M
  2. 1.2 × 10–4 M
  3. 2.2 × 10–4 M
  4. 1.1 × 10–10 M

Answer

(C) 2.2 × 10 –4 M

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