A 2.5 g impure sample of a weak monoacidic base (molar mass 45) is dissolved in 100 mL water and titrated with 0.5 M HCl at 25°C. At one-fifth neutralisation pH=9; at equivalence pH=4.5. Select correc

Question

A 2.5 g impure sample of a weak monoacidic base (molar mass 45) is dissolved in 100 mL water and titrated with 0.5 M HCl at 25°C. At one-fifth neutralisation pH=9; at equivalence pH=4.5. Select correct statements; log2=0.3.

TestHub · Accepted Answer

At one-fifth neutralisation [BH⁺]/[B]=1/4. pOH=5 gives pK_b=5-log(1/4)=5.6, so K_b=2.5×10⁻⁶ and A is false. K_a(BH⁺)=4×10⁻⁹. Equivalence pH=4.5 gives salt concentration ≈10⁻⁹/(4×10⁻⁹)=0.25 M, B. If initial base moles are n, n/(0.100+2n)=0.25, so n=0.050 mol and HCl volume=0.100 L, C. Base mass=0.050×45=2.25 g, or 90% rather than 80%. Answer B,C.

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