Question

Arrange the following 0.1M solutions in order of increasing pH and state why you placed each solution in that position: NaCH3COO, HCl, HCN, NaOH, NH3, NaCN, KNO3, H2SO4, NH4Cl, H2SO3, NaHCO3, Na3PO4, and CH3COOH.

Answer 1

pH of the solutions increases in the following range:

H2SO4 < HCl < H2SO3 < CH3COOH < HCN < NH4Cl < KNO3 < NaCH3COO < NaHCO3 < NaCN < NH3 < Na3PO4 < NaOH

There are three types of substances: acids, salts and bases.

Acids (pH<7): H2SO4, HCl, CH3COOH, H2SO3 and HCN
Salts: NaHCO3, NaCN, NaCH3COO, Na3PO4, KNO3 and NH4Cl
Bases (pH>7): NaOH and NH3 [ note: NH3 has no OH-groups but it is a base because in water solution it exists as NH4OH ( NH3 + H2O NH4OH ) ].

Acids and bases are divided into weak and strong. Strong acids dissotiate easily and, thus, have lower pH, while weak acids dissociate hardly and have higher pH. Strong bases dissociate easily and, thus, have higher pH, while weak bases dissociate hardly and have lower pH. NaOH is a strong base, that is why it is last in the range. NH4OH is a weak base.

The strength of the acids decrease in the range:

HCl > H2SO4 > H2SO3 > CH3COOH > HCN

If one doesn’t know which acid is stronger, one should find the pKa values for the acid: the greater the pKa the weaker the acid (note: HCl is stronger than H2SO4 but since the latter forms two hydrogen ions when dissociating it has lower pH).

pH of a salt solution depends on how the salt hydrolyze (if it does) in water solution. For such diluted solutions (0.1M) it may be assumed that salts hydrolyze completely. All salts are divided on:

- salts of strong acids and weak bases (so called acidic salts),
- salts of strong bases and weak acids (so called basic salts),
- salts of weak acids and weak bases,
- salts of strong acids and strong bases. NH4Cl is a salt of strong acid and weak base.

Salts of strong acids and weak bases hydrolyze producing strong acid. For NH4Cl:

NH4Cl + H2O NH4OH + HCl

Net ionic form: NH4⁺ + H2O NH4OH + H⁺

That is why this solution is acidic (pH < 7)

KNO3 is a salt of strong acid and strong base. Such salts don’t hydrolyze and their solution pH = 7. So, KNO3 solution has pH = 7.

There are no salts of weak acids and weak bases in the given list of salts.

NaHCO3, NaCN, NaCH3COO, Na3PO4 are salts of a strong base (NaOH) and weak acids (actually H3PO4 is not weak but medium acid).

Salts of strong bases and weak acids hydrolyze producing strong base. Equations for four such salts are as follows:

NaHCO3 + H2O H2CO3 + NaOH

Net ionic form: HCO₃^- + H2O H2CO3 + OH^-

NaCN + H2O HCN + NaOH

Net ionic form: CN- + H2O HCN + OH^-

NaCH3COO + H2O CH3COOH + NaOH

Net ionic form: CH3COO- + H2O CH3COOH + OH^-

Na3PO4 + 3 H2O H3PO4 + 3 NaOH

Net ionic form: PO₄^3- + 3 H2O H3PO4 + 3 OH^-

Since Na3PO4 forms three hydroxyl ions when hydrolyzing, its solution is most basic (even more basic them ammonia solution). All other salts form only one hydroxyl ion. The weaker the acid formed when salt hydrolyzes the more basic the solution is. The strength of the acids decreases in the range: CH3COOH > H2CO3 > HCN, so the pH of these three salts increases in the range: NaCH3COO < NaHCO3 < NaCN