Which of the two is the stronger base?Give your reason.
(a) HPO4 2- or PO4 3-
(b)HS- or S2-
I would look at it this way. PO4^3- as a base is
PO4^3- + HOH ==> HPO4^2- + OH^- and
Kb = (Kw/k3) = (1E-14/1E-13) = about 0.1. k3 for H3PO4 is about 1E-13; you can look it up in tables in your text.
You can see that most of the salt occurs in the form of HPO4^2- and not PO4^3- which means PO4^3- is the stronger base.
However, you can also calculate the HPO4^2- + HOH ==> H2PO4^- + OH^-
Kb HPO4^2- = (Kw/k2) = 1E-14/1E-8 = about 1E-6 which means most of the salt stays as HPO4^2-. The 1E-8 for k2 is from memory; again, yo can look up the exact number in your tables.
HS^-/S^2- is done the same way.
(a) To determine which of the two is the stronger base, we need to examine their respective conjugate acids and consider their acid dissociation constants (pKa) values.
The conjugate acid of HPO4 2- is H2PO4-, and the conjugate acid of PO4 3- is HPO4 2-. Comparing these acids, we can see that H2PO4- has a lower pKa value.
Generally, the lower the pKa value of an acid, the stronger the acid is, and consequently, the weaker its conjugate base will be. Therefore, HPO4 2- is a weaker base than PO4 3-.
(b) Similarly, we need to compare the conjugate acids and their respective pKa values to determine which of the two, HS- or S2-, is the stronger base.
The conjugate acid of HS- is H2S, and the conjugate acid of S2- is HS-. Looking at the acids, it can be observed that H2S has a lower pKa value.
Following the same logic as above, since HS- is the conjugate base of a stronger acid (H2S), HS- is a weaker base compared to S2-.
To determine which of the two is the stronger base, we need to consider their conjugate acids and compare their acid dissociation constants (Ka values).
(a) HPO4 2- or PO4 3-:
- HPO4 2- is the conjugate base of H2PO4-, which is a weak acid.
- PO4 3- is the conjugate base of H3PO4, which is a strong acid.
To determine the stronger base, we compare the Ka values. Since H3PO4 is a stronger acid (larger Ka value) compared to H2PO4-, PO4 3- will be a stronger base compared to HPO4 2-. This is because the stronger the acid, the weaker its conjugate base.
(b) HS- or S2-:
- HS- is the conjugate base of H2S, which is a weak acid.
- S2- is the conjugate base of H2S2, which is a weak acid.
Since both H2S and H2S2 are weak acids, we cannot determine which of the two is the stronger base based on their Ka values alone. In this case, we need to consider other factors such as electronegativity and atomic size to determine which is the stronger base.