SECTION 8–4:STRENGTH OF ACIDS AND BASESPhysical Science

Coach Kelsoe

Pages 246–249

OBJECTIVES

Define pH, and relate pH to hydronium ion concentration in a solution.

Distinguish between strong acids and weak acids, and between strong bases and weak bases.

Define buffer, and describe how a buffer can be prepared.

Explain how electrolytes can be classified.

THE PH SCALE

Chemists use a number scale from 0 to 14 to describe the concentration of hydronium ions in a solution. It is known as the pH scale.

The pH of a solution is a measure of its hydronium ion concentration. A pH of 7 indicates a neutral solution. Acids have a pH less than 7. Bases have a pH greater than 7.

The lower the pH value, the greater the H3O+ ion concentration in solution is.

The higher the pH value, the lower the H3O+ ion concentration is.

THE PH SCALE

Distilled water has a hydronium ion concentration of 1 x 10-7 M. What is the pH of a distilled water? A: 7.

The pH of a solution is 7. The concentration of hydronium ions in the solution is 1 x 10-7 M.

The pH of a solution describes the concentration of hydronium ions in solution.

The hydroxide ion concentration in pure water at 25°C is NOT equal to ___. A: 1 x 107 M.

THE PH SCALE

A compound has a pH of 6 in solution, where very little of it has ionized. The compound is a ___. A: weak acid.

Calcium hydroxide forms very few ions in solution, but it is still considered a strong base because ___. A: all the calcium hydroxide in solution dissociates.

STRONG AND WEAK ACIDS AND BASES

When certain acids and bases dissolve in water, the formation of ions from the solute almost goes to completion. Such acids and bases are classified as strong.

Weak acids and bases ionize or dissociate only slightly in water.

Acids that contain carbon are weak acids, which means that they ionize only to a small extent in solution.

Many foods are acidic. These foods do not hurt you when you eat them because they contain weak acids.

BUFFERS Weak acids and bases can be used to make

buffers. A buffer is a solution that is resistant to large

changes in pH. Buffers can be prepared by mixing a weak acid

and its salt or a weak base and its salt. Because a buffer can react with both an acid

and a base, its pH remains relatively constant. Which of the following would NOT form a buffer

solution? A: HCl + NaCl A small amount of acid is added to a buffer

solution. The pH of the solution will ___. A: stay about the same.

ELECTROLYTES An electrolyte is a substance that ionizes or

dissociates into ions when it dissolves in water. The resulting solution can conduct electric current.

A substance that ionizes or dissociates into ions when placed in water is always a(an) ___. A: electrolyte.

The electrolytes in sports drinks help restore the balance of ions in your body.

Electrolytes can be classified as strong or weak. Strong acids and bases are strong electrolytes because

they dissociate or ionize almost completely in water. Which of the following are examples of strong

electrolytes? A: strong acids and strong bases. Acetic acid is considered a weak electrolyte because it

only partially ionizes.

VOCABULARY

pH Buffer Electrolyte

STUDY GUIDE QUESTIONS

#22-29, 44-48 should be answered.