Phosphoric acid is a triprotic acid:
H3PO4(aq) +H2O(l) ↔ H3O+(aq) + H2PO4-(aq) Ka1 = 7.5 x 10-3
H2PO4-(aq) +H2O(l) ↔ H3O+(aq) + HPO42-(aq) Ka2 = 6.2 x 10-8
HPO42-(aq) +H2O(l) ↔ H3O+(aq) + PO43-(aq) Ka3 = 4.8 x 10-13
Determine whether sodium monohydrogen phosphate (Na2HPO4) is neutral, basic, or acidic.
First, what is its Ka when it acts as an acid?
Second, what is its Kb when it acts as a base?
HPO4^2- + HOH ==> H2PO4^- + OH^- as a base since it adds a H^+..
HPO4^2- + H2O --> PO4^3- + H3O^+ as an acid since it donates a H^+.
Ka2 for HPO4^2- = 6.2E-8 from the data so Kb = Kw/Ka = 10E-15/6.2E-8 = 1.6E-7
Which is the larger number?
FYI. Solutions of Na2HPO4 have a pH between 8.0 and 11.0
To determine whether sodium monohydrogen phosphate (Na2HPO4) is neutral, basic, or acidic, you can consider the equilibrium reactions involving the phosphate ion.
1. Ka when Na2HPO4 acts as an acid:
The compound Na2HPO4 can donate a proton (H+) to water, and the resulting ion is HPO42-. This can undergo further ionization to form PO43-. Therefore, we need to consider the dissociation of HPO42- to determine its acidity.
HPO42-(aq) + H2O(l) ↔ H3O+(aq) + PO43-(aq)
The Kb expression is derived from the reaction above, and it is as follows:
Kw = [H3O+][PO43-] / [HPO42-]
However, since we need to find the Ka when Na2HPO4 acts as an acid, we can simply take the reciprocal of Kb.
Ka = 1 / Kb
So, to find the Ka for Na2HPO4 acting as an acid, we need to calculate the Kb for HPO42-.
2. Kb when Na2HPO4 acts as a base:
The compound Na2HPO4 can accept a proton (H+) from water, and the resulting ion is H2PO4-. This can undergo further ionization to form H3PO4. Therefore, we need to consider the dissociation of H2PO4- to determine its basicity.
H2PO4-(aq) + H2O(l) ↔ H3O+(aq) + HPO42-(aq)
The Kb expression is derived from the reaction above, and it is as follows:
Kw = [H3O+][HPO42-] / [H2PO4-]
To find the Kb for Na2HPO4 acting as a base, we need to calculate the concentration of H2PO4-.
Once you have the concentration of H2PO4-, you can use the Kw expression to calculate its Kb.
To determine whether sodium monohydrogen phosphate (Na2HPO4) is neutral, basic, or acidic, we need to consider its acidic and basic properties.
1. Ka when Na2HPO4 acts as an acid:
First, let's look at the formula of Na2HPO4. It contains the HPO42- ion, which can act as an acid by donating a proton (H+). The equilibrium reaction for this acid dissociation is as follows:
HPO42-(aq) + H2O(l) ↔ H3O+(aq) + PO43-(aq)
Since we don't have a direct Ka value for Na2HPO4, we can use the Ka values of phosphoric acid (H3PO4) to calculate an approximate Ka value for Na2HPO4 acting as an acid. This can be done by multiplying the Ka values together along the reaction path.
Ka(Na2HPO4) = Ka1 × Ka2
Given:
Ka1 = 7.5 × 10^-3
Ka2 = 6.2 × 10^-8
Calculating Ka(Na2HPO4):
Ka(Na2HPO4) = (7.5 × 10^-3) × (6.2 × 10^-8)
Using these values, we can calculate the approximate Ka when Na2HPO4 acts as an acid.
2. Kb when Na2HPO4 acts as a base:
The formula of Na2HPO4 also contains the H2PO4- ion, which can act as a base by accepting a proton (H+). The equilibrium reaction for this base dissociation is as follows:
H2PO4-(aq) + H2O(l) ↔ H3O+(aq) + HPO42-(aq)
To calculate the Kb value for Na2HPO4, we can use the Kb values corresponding to the Ka values we used earlier. The relationship between Ka and Kb is given by the equation:
Kw = Ka × Kb
Kw represents the ionization constant of water, which is approximately 1.0 × 10^-14 at 25°C.
Rearranging the equation, we can solve for Kb:
Kb = Kw / Ka
Using this equation, we can calculate the Kb value when Na2HPO4 acts as a base.
Remember to use the given Ka values for phosphoric acid and solve for Ka(Na2HPO4) and Kb(Na2HPO4) accordingly.