AP Chemistry Unit 8 - pH of salts

UNIT 8 - Acids and Bases from SALTS

Can be neutral molecules or anions.

the fine print:

the negative ions of some multiprotic acids are

acids not bases (HC8H4O4⁻), like KHP in our lab

Bases

can be neutral molecules or cations.

​Acids

Acids and Bases

Anions of strong acids...

DO NOT CHANGE pH!!

If the anion of a strong acid would interact with the water the resulting compound just completely dissociates.

For example, hydrochloric acid is a strong acid:
HCl --> H⁺ + Cl^-

Cl^- + H₂O --> HCl + OH^- more accurately Cl^- + H₂O --> H⁺ + Cl^- + OH^-

The H+ and OH- would just recombine to form H₂O. NO CHANGE in pH!

Cations of strong bases...

DO NOT CHANGE pH!!

If the cation of a strong base would interact with the water the resulting compound completely dissociates.

For example:
KOH --> K⁺ + OH^-

K⁺ + H₂O --> KOH + H⁺ more accurately K⁺ + H₂O --> K⁺ + OH^- + H⁺

The H⁺ and OH^- would just recombine to form H₂O. NO CHANGE in pH!

unless we have the anion of a multiprotic acid.

nearly all other - ions are bases

all other + ions are acids

So....memorize your strong acids and bases.

Multiple Choice

When CsI is dissolved in water, will the pH go up, down, or stay at 7?

• pH will go up

• pH will go down

• pH will stay at 7

Multiple Choice

When HBr is dissolved in water, will the pH go up, down, or stay at 7?

• pH will go up

• pH will go down

• pH will stay at 7

Multiple Choice

When KCN is dissolved in water, will the pH go up, down, or stay at 7?

• pH will go up

• pH will go down

• pH will stay at 7

Multiple Choice

When HBrO₂ is dissolved in water, will the pH go up, down, or stay at 7?

• pH will go up

• pH will go down

• pH will stay at 7

Multiple Choice

When NH₄NO₃ is dissolved in water, will the pH go up, down, or stay at 7?

• pH will go up

• pH will go down

• pH will stay at 7

Multiple Choice

When NaOH is dissolved in water, will the pH go up, down, or stay at 7?

• pH will go up

• pH will go down

• pH will stay at 7

Multiple Choice

When NH₃ is dissolved in water, will the pH go up, down, or stay at 7?

• pH will go up

• pH will go down

• pH will stay at 7

Multiple Choice

When H₂CO₃ is dissolved in water, will the pH go up, down, or stay at 7?

• pH will go up

• pH will go down

• pH will stay at 7