Question

The flocculation value of HCl for arsenic sulphide sol.
is 30 m mol L^-1 If H₂SO₄ is used for the flocculatiopn of arsenic sulphide, the amount in grams, of H₂SO₄ in 250 ml required for the above purposed is ______.
(molecular mass of H₂SO₄ = 98 g/mol)

Hint:

As₂S₃ is a negative sol, so it is coagulated by the cation H⁺; the same concentration of H⁺ is needed whichever acid supplies it.

Answer 1

Correct Answer: 0.37

Step 1: Arsenic sulphide sol is negatively charged, so the coagulating ion is H⁺.
Step 2: HCl flocculation value = 30 mmol/L, i.e. 30 mmol/L of H⁺ is required.
Step 3: H₂SO₄ gives 2 H⁺ per molecule, so the required H₂SO₄ concentration is 15 mmol/L.
Step 4: In 250 mL: \(15\times0.25=3.75\) mmol \(=3.75\times10^{-3}\) mol.
Step 5: Mass \(=3.75\times10^{-3}\times98=0.3675\) g \(\approx0.37\) g (accepted range 0.36 to 0.38).
Answer: 0.37