Chemistry · Chapter 14

Same Concentration, Different Acid

Pour the same amount of acid into two beakers. One is strong and one is weak. Watch how far each one actually ionizes, and what that does to the pH.

Two beakers 100 particles drawn in each

A strong acid and a weak acid at the same concentration

Choose the weak acid

Acetic acid at 0.100 mol/L reaches pH 2.88. The strong acid reaches pH 1.00.

Side by side

MEASURE HCl (STRONG) ACETIC (WEAK)
pH1.002.88
[H3O+] mol/L1.00e-11.33e-3
Percent ionized1001.33
Intact acid left09.87e-2
1.88pH units apart 76x in H3O+

How the pH is found

Strong: ionizes fully, so [H3O+] equals the concentration.
Weak: solve Ka = x2 ÷ (C − x) for x.
Here: 1.8e-5 = x² ÷ (0.100 − x)
Exact x: 1.33e-3
Shortcut √(KaC): 1.34e-3
Shortcut safe? yes, under 5 percent

pH bars

pH of the two solutions compared

Dilute both acids and the gap shrinks. A weak acid ionizes a larger share when there is more room.