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260 ACID-BASE THEORY [CHAP. 17
(b) Let [H 3 O ] = x.
+
+
−
[H 3 O ][A ] −2
K a = = 8.0 × 10
[HA]
x 2
= 8.0 × 10 −2
0.10 − x
−2
2
x = 8.0 × 10 (0.10 − x)
Here the x cannot be neglected when subtracted from 0.10, because the equilibrium constant is too big.
The problem is solved by using the quadratic formula (Appendix).
x = 5.8 × 10 −2
