Definition of pH

State the definition for pH

Give the pH for a solution when given the molarity and access to a calculaltor

Write the equation for the process of dissolving a base in water

Give numbers and identities for ions formed when acids and bases dissolve in water

Explain what an is meant by an aqueous solution

Classify a substance as an acid or base by examination of the formulas

Objectives

Fundamental ideas

The acid potential for an aqueous solution is indicated by the pH.

The pH scale is exponential. A change of pH by "1" unit reflects a ten-fold concentration change.

The pH scale is connected to the concentration of hydronium ions. The pH is low for solutions rich in hydronium ions. Temperature influences pH. Water solutions at 25oC are acidic when the pH is lower than 7.

Solutions at 25oC are considered neutral when the pH equals 7.

 

Definitions

 

sodium hydroxide
water

sodium chloride

hydrochloric acid
HCl(aq)
+ NaOH(aq)
---ý
H2O(l)
+ NaCl(aq)

The hydrogen ion, H+ , does not exist in water. The lone pairs of electrons on the oxygen of the water molecule are attracted to the hydrogen ion, H+.

Hydrogen chloride hydronium ion chloride ion

HCl(g) + H-O-H --ý H3O+ (aq) + Cl-(aq)

In water the hydronium ion is the active agent formed by acids.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Substances will be acidic if a hydrogen atom has a weak bond to a nonmetal atom. Typically this means the hydrogen will be bonded to one of the halogens, oxygen, or sulfur.

 

 

 

 

 

Nitric acid

potassium hydroxide

 

water

potassium nitrate

 

HNO3(aq)

+ KOH(aq)

---ý

H2O(l)

+ KNO3(aq)

 

 

 

 

 

 

 

 

 

 

Fundamental ideas

 

 

 

 

 

 

 

 

 

 

 

Bases are a source of OH1-

 

Potassium hydroxide potassium ions

KOH(s) + H2O(l) -----ý K1+ (aq) + OH1- (aq)

 

Some substances react with water to form hydroxide ions.

NH3 (s) + H2O(l) -----ý NH41+ (aq) + OH1- (aq)

 

 

 

Exercise:

Label the following as an acid or a base. When dissolved in water,. Click on formula to see answers. Give the formulas for the ions formed by each substance in aqueous solution.

Al(OH)3(s) HBr(g) Ca(OH)2(s) Fe(OH)3 (s)

 

 

Answer.

Al(OH)3(s) aluminum hydroxide dissolves in water and acts a base

Al(OH)3(s) + H2O(l) -----ý Al3+ (aq) + 3 OH1- (aq)

 

HBr(g) dissolves in water to form and acts as an acid.

HBr(g) + H-O-H --ý H3O+ (aq) + Br-(aq)

 

 

 

Ca(OH)2(s) calcium hydroxide dissolves in water and acts a base

Ca(OH)2(s) + H2O(l) -----ý Ca2+ (aq) + 2 OH1- (aq)

 

 

Fe(OH)3(s) ironIII hydroxide dissolves in water and acts a base

Fe(OH)3(s) + H2O(l) -----ý Fe3+ (aq) + 3 OH1- (aq)