Answer
6(b)
$$[H_3O^+] = 2.2 \times 10^{-13} \space M$$
7(a)
$$[H_3O^+] = 4.8 \times 10^{-13} \space M$$
8(b)
$$[H_3O^+] = 8.5 \times 10^{-13} \space M$$
Work Step by Step
6(b)
$$[H_3O^+] = \frac{1.0 \times 10^{-14}}{[OH^-]} = \frac{1.0 \times 10^{-14}}{ 0.045 } = 2.2 \times 10^{-13} \space M$$
7(a)
$$[H_3O^+] = \frac{1.0 \times 10^{-14}}{[OH^-]} = \frac{1.0 \times 10^{-14}}{ 0.0210 } = 4.8 \times 10^{-13} \space M$$
8(b)
$$[H_3O^+] = \frac{1.0 \times 10^{-14}}{[OH^-]} = \frac{1.0 \times 10^{-14}}{ 0.0117 } = 8.5 \times 10^{-13} \space M$$
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All these concentrations are lower than $[H_3O^+] = 1.0 \times 10^{-7}$, which makes sense, since they are all solutions made of basic compounds, and therefore, basic solutions.
