Question

1. The table below is the experimental pH values for the solutions of 0.1M. Use the table to answer the following questions:

Solution (0.1 M)	Observed pH
Sodium Acetate	7.35
Calcium Formate	6.65
Sodium Hydrogenate Phosphate	9.03
Formic Acid	2.57
Acetic Acid	2.88
Sodium Dihydrogen Phosphate	4.48
Hydrochloric Acid	1.44
Hydrobromic Acid	1.37
Nitric Acid	1.35
Sodium Nitrate	4.18
Sodium Chloride	5.25
Sodium Bromide	5.45
DI Water	6.50
Tap Water	5.75

a)Which set of solutions that you measured are strong acids? Explain your choice based on the observed pH.

b)Which set of solutions that you measured are the cconjugate bases of strong acids? Explain your choice based on the observed pH.

c)Which set of solution are the weak acids? Explain your choice based on the observed pH.

d) Which set of solutions that you measured are the conjugate bases of the weak acids? Explain youe choice based on the observed pH.

Answer 1

The pH is the measure of the strength of the acid in the solution. It is given as,

The strong acid dissociate completely in the solution and thus it has high concentration of H+ ions. Since the cocentration of H+ ions is large, therefore, the -log[H+] will be small a value.

Thus for pH values of solution having concentration 0.1M, the pH value close to 1 is the strong acids. Further, pH + pOH=14, thus pH value close to 13 will be consider as the strong base and pH value close to 7 is neutral.

Also in principle, in a conjugate acid-base pair, the conjugate of stron acid is a weak base and the conjugate of weak acid is the strong base. THus from the given values of pH of 0.1 M solution, we can draw the following concluions.

Note that, these are classified based on the p^H vaues given in the above table.

Ans (a):Stron Acids

The set of solutions that have strong acids are those which have pH value equal or close to 1, since it has high concentration of H+ ions in the solution.

Thus the strong acids among list are and their trend of acididty is-

HNO₃ (p^H=1.35)> HBR (p^H=1.37)> HCl (p^H=1.44)

Ans (b): Conjugate Bases pair of Strong Acids

The conjugate bases of the strong acids are the weak bases, thus from the above list the conjugates bases having small pOH (large pH) are the congugate pairs of strong acid, which among list are

sodium hydrogen phospate (p^OH=4.97)>sodium acetate (p^OH=6.65)>calcium formate(p^OH=7.35).

Ans (c): Weak Acids

Weak acids are those acids which have very small dissociation in solution and thus have large pH but smaller than nutral value. Thus the acids amomng list are,

DI water (p^H=6.50)>Tap water (p^H=5.75)>acetic acid (p^H=2.88)>formic acid(p^H=2.57)

Ans (d): Conjugate bases of Weak Acids

The conjugate bases of the weak acids are the strong bases, thus from the above list the conjugates bases having large pOH (small pH) are the congugate pairs of waek acids. thus from the above list conjugate pairs of weak acids are,

sodium nitrate (p^OH=9.82) > sodium dihydrogen phosphate (p^OH=9.52)> sodium chloride (p^OH=8.75)>sodium bromide (p^OH=8.55)