First, write the equilibrium expression: HA ⇌ H⁺ + A⁻. Then, take initial concentration as 'C', assume 'x' is small, so x ≈ √(Ka*C) to find [H⁺] and then pH = -log[H⁺]. You see, this approximation works like my scooter battery—it holds only if the drain isn't too much, meaning Ka must be small and C not too tiny. I know because I'm always calculating if my battery will last the route, just like you check if 'x' is less than 5% of C to be sure.
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