The pH scale | Acids and bases | AP Chemistry | Khan Academy

Summary

This video explains how to calculate pH and its significance in aqueous solutions. It covers the relationship between hydronium ion concentration and pH, with examples such as pure water, lemon juice, and a cleaning solution containing ammonia. The script demonstrates the use of logarithms to determine pH values and discusses the pH scale, from acidic to basic solutions. It also introduces the concept of pOH and the equation pH + pOH = 14 at 25°C, highlighting the balance between hydronium and hydroxide ion concentrations. The video concludes by reinforcing the importance of pH in understanding solution properties.

Takeaways

• 😀 pH is calculated as the negative logarithm of the concentration of hydronium ions (H₃O⁺).
• 😀 Pure water at 25°C has a pH of 7.00, which is considered neutral.
• 😀 The pH of an acidic solution is less than 7, while the pH of a basic solution is greater than 7.
• 😀 For lemon juice, with a hydronium ion concentration of 3.6 × 10⁻⁴ M, the pH is calculated to be 3.44.
• 😀 pH is a logarithmic scale, meaning a 1-unit change in pH represents a tenfold change in ion concentration.
• 😀 For a cleaning solution with ammonia, the hydroxide ion concentration was 2.0 × 10⁻³ M, leading to a pH of 11.30.
• 😀 pOH is the negative logarithm of the hydroxide ion concentration, and pH + pOH = 14.00 at 25°C.
• 😀 The concentration of hydronium and hydroxide ions in water are related through the Kw equation, where their product equals 1.0 × 10⁻¹⁴ at 25°C.
• 😀 The pH scale is typically from 0 to 14, but it can extend beyond these limits in certain cases.
• 😀 The pH of a solution can be estimated by comparing it to known concentrations of ions and applying logarithmic principles.

Q & A

What is the relationship between pH and the concentration of hydronium ions in a solution?

pH is the negative logarithm of the concentration of hydronium ions in a solution. It is calculated using the formula: pH = -log[H₃O⁺], where [H₃O⁺] is the concentration of hydronium ions.

What is the pH of pure water at 25 degrees Celsius?

The pH of pure water at 25 degrees Celsius is 7.00. This is because the concentration of hydronium ions in pure water is 1.0 × 10⁻⁷ M.

How do significant figures affect the calculation of pH?

The number of significant figures in the concentration of hydronium ions determines the number of decimal places in the final pH value. For example, if the concentration has two significant figures, the pH will have two decimal places.

How can you estimate the pH of a solution without a calculator?

To estimate the pH, compare the concentration of hydronium ions to known values like 1 × 10⁻⁴ M or 1 × 10⁻³ M. For example, if the concentration is between these values, the pH will fall between 3 and 4.

How do you calculate the pH of a solution with hydroxide ions?

To calculate the pH of a solution with hydroxide ions, first calculate the concentration of hydronium ions using the equation Kw = [H₃O⁺][OH⁻]. Once you have the hydronium ion concentration, use the formula pH = -log[H₃O⁺] to find the pH.

What is the pH of a cleaning solution with ammonia if the hydroxide ion concentration is 2.0 × 10⁻³ M?

The pH of the cleaning solution with ammonia is 11.30. This is calculated by first finding the concentration of hydronium ions using the Kw equation, and then applying the pH formula.

What is the pH scale, and how is it used to classify substances?

The pH scale ranges from 0 to 14, with pH 7 being neutral. A pH less than 7 indicates an acidic solution, while a pH greater than 7 indicates a basic solution. The lower the pH, the more acidic the solution; the higher the pH, the more basic the solution.

What does a pH of 5 indicate in terms of acidity?

A pH of 5 indicates a mildly acidic solution. It is 10 times more acidic than a solution with a pH of 6, based on the logarithmic nature of the pH scale.

What is the pOH, and how does it relate to pH?

pOH is the negative logarithm of the hydroxide ion concentration, similar to how pH relates to the hydronium ion concentration. The relationship between pH and pOH is given by the equation pH + pOH = 14.00 at 25 degrees Celsius.

What is the significance of Kw, and how does it relate to the pH and pOH of a solution?

Kw is the ionization constant for water, equal to 1.0 × 10⁻¹⁴ at 25°C. It is used to calculate the concentrations of hydronium and hydroxide ions in a solution. The pH and pOH are related to Kw through the equation pH + pOH = 14.00.