acid and base reactions?

_________NH₄⁺(aq) ⇌ NH₃(aq) + H⁺(aq) ___ Kₐ = 5.6 × 10⁻¹⁰
Initial: ____ 1.26 M ___ 0 M _____ 0M
Change: ___ -y M ____ +y M ___ +y M
At eqm: _ (1.26 - y) M __ y M ____ y M

As Kₐ is small, the extent of dissociation of NH₄⁺ ions is very small.
Hence, [NH₄⁺] at eqm = (1.26 - y) M ≈ 1.26 M

Kₐ = [NH₃] [H⁺] / [NH₄⁺]
5.6 × 10⁻¹⁰ = y² / 1.26
y = √(1.26 × 5.6 × 10⁻¹⁰) = 2.7 × 10⁻⁵
[H⁺] = 2.7 × 10⁻⁵ M

The answer: 2.7 × 10⁻⁵ M


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HA(aq) ⇌ H⁺(aq) + A⁻(aq) ____ Kₐ = [H⁺] [A⁻] / [HA]

The answer: Kₐ = [H⁺] [A⁻] / [HA]