Exercises
Exercise 1. A shampoo has [H3O+] = 3.2 x 10-8 M. What is the hydroxide ion concentration at 25°C? Is the shampoo acidic, neutral, or basic?
\(\displaystyle [OH^-]\;=\;\frac{1.0\times 10^{-14}}{3.2\times 10^{-8}\;M}\;=\;\mathbf{3.1\times 10^{-7}\;M}\)
The shampoo is basic.
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Exercise 2. What are the concentrations of H3O+ and OH– in 0.16 M Ba(OH)2 at 25°C? Is the solution acidic, basic, or neutral?
There are two moles of OH– in one mole of Ba(OH)2. The concentration of OH– is 0.16 M x 2 = 0.32 M
The concentration of H3O+ can be found using Kw
\(\displaystyle [H_3O^+]\;=\;\frac{1.0\times 10^{-14}}{0.32\;M}\;=\;\mathbf{3.1\times 10^{-14}\;M}\)
The solution is basic.
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Exercise 3. At 25°C, Kw is 1.0 x 10-14. What is Kw at 30°C if the hydroxide concentration of pure water at this temperature is 1.2 x 10-7 M?
At 30°C the concentrations of hydronium ion and hydroxide ion are equal. Therefore,
\(\displaystyle K_w\;=\;[H_3O^+][OH^-]\;=\;[1.2\times 10^{-7}\;M][1.2\times 10^{-7}\;M]\;=\;\mathbf{1.4\times 10^{-14}}\) at 30°C.
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Exercise 4. What is the hydronium ion and hydroxide ion concentrations of 0.025 M HNO3 at 25°C?
HNO3 is a strong acid, therefore, the concentration of H3O+ is 0.025 M. The concentration of OH– is
\(\displaystyle [OH^-]\;=\;\frac{1.0\times 10^{-14}}{0.025}\;=\;\mathbf{4.0\times 10^{-13}\;M}\)