Consider the diprotic acid H2A with K1 = 1.00 x 10-4 and K2 = 1.00 x 10-8 . Find the pH and concentrations of H2A, HA-, and A2- in (a) 0.100 M H2A; (b) 0.100 M NaHA; (c) 0.100 M Na2A

Explanation:

H₂A    ⇄   HA⁺  +  A⁻

K₁ = [ HA⁺] [A⁻ ] / [ H₂A ]

HA⁺ ⇄  H⁺ + A⁻

K₂ = [ H⁺] [ A ⁻ ] / [ HA⁺ ]

H₂A           ⇄   HA⁺      +        A⁻

.100                     0                 0

.100 - C                C                C

K₁ = [ HA⁺] [A⁻ ] / [ H₂A ]

Putting the values

1.00 x 10⁻⁴ =   C X C / .1  -  C

10⁻⁵ - 10⁻⁴ C = C²

C²  + 10⁻⁴ C - 10⁵ = 0

C = 8 X 10⁻² M

So concentration of H₂A that is [ H₂A ]  = 1 - C =   0 .1 - 8 X 10⁻² M

= .02 = 2 x 10⁻² M

NaHA    ⇄   Na⁺ + HA⁻

NaHA is a strong acidic salt  so it will ionise 100 %

NaHA    ⇄   Na⁺ + HA⁻

.1                   .1         .1  

concentration of HA⁻  =  .1 M

Na₂A ⇄  2Na⁺  + A⁻²

Na₂A is also a strongly ionic  salt so it will dissociate 100 % .

Na₂A ⇄  2Na⁺  + A⁻²

.1               .1          .1  

concentration of A⁻²  = .1 M  

HA⁺    ⇄       H⁺   +     A⁻

8 X 10⁻²         C₁        C₁  

K₂ = [ H⁺] [ A ⁻ ] / [ HA⁺ ]

Putting the values

1.00 x 10⁻⁸ = C₁ X C₁ /  C

1.00 x 10⁻⁸ = C₁²  /  8 X 10⁻²

C₁² = 8 x 10⁻¹⁰

C₁ = 2.828 x 10⁻¹⁰

[ H⁺]  = 2.828 x 10⁻¹⁰

pH = - log  [ H⁺]  = - log 2.828 x 10⁻¹⁰

= 10 - log 2.828

= 10 - .45

= 9.55 .

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.