Question

Using Table 7.3, order the following acids from strongest to weakest. HNO₃, H₂O, NH₄⁺, and C₅H₅NH⁺.

Short answer

Order of acidic strength from strongest to weakest is

HNO₃> C₅H₅NH⁺ > NH₄⁺ > H₂O

Step-by-step solution

Strong Acids

A strong acid is a acid which can easily give [H⁺] ions and take [OH^-] ions from base in an acid–base reaction. The examples of strong acids are HNO₃, HCl and H₂SO₄ etc.

Acid strength is given by K_a and base strength is given by K_b (base dissociation constant) values, stronger the K_b value stronger will be the base and stronger the base weaker will be its conjugate acid.

So we can conclude that smaller the K_b, stronger will be the conjugate acid.

Checking Strength

HNO₃ is a strong acid and H₂O is the conjugate acid of strong base OH^-.As discussed, strong bases have the weakest conjugate acids. So, H₂O is a weak conjugate acid.

C₅H₅N and NH₃ are both weak bases, therefore, will have strong conjugate acids. K_b values of these two are given in the table 7.3 are

Base	Formula	Conjugate Acid	Kb
Ammonia	NH3	NH4+	1.8×10-5
Pyridine	C5H5N	C5H5NH+	1.7×10-9

As the K_b value of C₅H₅N <NH₃

C₅H₅NH⁺will be a stronger conjugate acid than NH₄⁺.

Thus, overall order comes out to be

HNO₃> C₅H₅NH⁺ > NH₄⁺ > H₂O