1) Calculate [H3O+] in each aqueous solution at 25C, and classify each solution

Question

1)

Calculate [H3O+] in each aqueous solution at 25C, and classify each solution as acidic or basic.

Part A

[OH]= 5.2×104 M

Express your answer using two significant figures.

Part B

[OH]= 1.4×1012 M

Express your answer using two significant figures.

2)

Part A

0.50 g of hydrogen chloride (HCl) is dissolved in water to make 8.0 L  of solution. What is the pH of the resulting hydrochloric acid solution?

Express the pH numerically to two decimal places.

Part B

0.60 g of sodium hydroxide (NaOH) pellets are dissolved in water to make 7.5 L of solution. What is the pH of this solution?

Express the pH numerically to two decimal places.

3)

What assumption is commonly used to simplify the process of finding the pH for a solution of a weak acid?

What assumption is commonly used to simplify the process of finding the pH for a solution of a weak acid?

The concentration of H3O+ is assumed to be directly found from the K a expression, so all that is needed is to plug the initial concentrations of each species into the K a expression.

4)

A certain weak acid, HA, has a Ka value of 8.9×107.

Part A

Calculate the percent ionization of HA in a 0.10 M solution.

Express your answer to two significant figures and include the appropriate units.

Part B

Calculate the percent ionization of HA in a 0.010 M solution.

Express your answer to two significant figures, and include the appropriate units.

The concentration of H3O+ in solution is assumed to be equal to the concentration of the weak acid. For many weak acid solutions, it is assumed that [H3O+] is small relative to the initial concentration of acid, so the "x is small" approximation is used. The quadratic equation is the fastest way to find the pH of a weak acid, and the assumption is that the quadratic equation will work.

The concentration of H3O+ is assumed to be directly found from the K a expression, so all that is needed is to plug the initial concentrations of each species into the K a expression.

4)

A certain weak acid, HA, has a Ka value of 8.9×107.

Part A

Calculate the percent ionization of HA in a 0.10 M solution.

Express your answer to two significant figures and include the appropriate units.

Part B

Calculate the percent ionization of HA in a 0.010 M solution.

Express your answer to two significant figures, and include the appropriate units.

Explanation / Answer

1)

Calculate [H3O+] in each aqueous solution at 25C, and classify each solution as acidic or basic.

Part A

[OH]= 5.2×104 M

By definition, in water

Kw = [H+][OH]

Kw = 10^-14 always at 25°C

[H+] = Kw/[OH-] = (10^-14) / 5.2×104 M = 1.923*10^-11

[H+] =1.923*10^-11 which is basic since H+ ions is too low (will be pH around 11)

Express your answer using two significant figures.

Part B

[OH]= 1.4×1012 M

Apply same logic as in PArt A

Kw = [H+][OH-]

[H+] = Kw/[OH-]

[H+] = (10^-14) / (1.4×1012 M) = 0.00714

[H+] = 0.00714 which is acidic due to high concnetration

Express your answer using two significant figures.

NOTE: consider posting questions 2 and 3 in another set of Q&A. We are not allowed to answer to multiple set of Q&A

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