The concentration of ammonia solution (K_b=1.8x10^-5) that has pH 11 at 298 K is nearest to (neglect [OH-] against c)

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The concentration of ammonia solution ( $K_{b} = 1.8 \times 1 0^{- 5}$ ) that has pH 11 at 298 K is nearest to (neglect $[OH X^{-}]$ against $c$ )

pH 11 gives

[OH X^{-}] = 1 0^{- 3} M

. From

K_{b} = [OH X^{-}]^{2} / c

c = 1 0^{- 6} / (1.8 \times 1 0^{- 5}) = 0.056 M