S ECTION 8–4: S TRENGTH OF A CIDS AND B ASES Physical Science Coach Kelsoe Pages 246–249

O BJECTIVES 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.

T HE P H S CALE 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 H 3 O + ion concentration in solution is. The higher the pH value, the lower the H 3 O + ion concentration is.

T HE P H S CALE Distilled water has a hydronium ion concentration of 1 x 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 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 10 7 M.

T HE P H S CALE 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.

S TRONG AND W EAK A CIDS AND B ASES 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.

B UFFERS 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.

E LECTROLYTES 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.

V OCABULARY pH Buffer Electrolyte

S TUDY G UIDE Q UESTIONS #22-29, should be answered.