Question

Classify each substance as a strong acid, strong base, weak acid, or weak base.

Answer 1

Answer:

HCOOH  (formic acid)     weak acid

HClO₄                              strong acid

HI                                      strong acid

NH₃                                   weak base

NaOH                                strong base

HNO₂                                weak acid

CH₃COOH (acetic acid)   weak acid

(CH₃)₂NH                           weak base

HNO₃                                strong acid

Ca(OH)₂                            strong base

HCl                                     strong acid

KOH                                   strong base

HCN                                   weak acid

HF                                      weak acid

CH₃NH₂                             weak base

CsOH                                 strong base

HBr                                    strong acid

LiOH                                  strong base

Ba(OH)₂                            strong base

H₂SO₄                               strong acid

Explanation:

Strong electrolytes are those that completely dissociate in water, so they exist mostly in the ionic form.

AB(aq) ⇒ A⁺(aq) + B⁻(aq)

On the other hand, weak electrolytes dissociate only partially in water, so that at equilibrium there is a mixture of the molecular form and the ionic form.

AB(aq) ⇄ A⁺(aq) + B⁻(aq)

Electrolytes can be acid or basic.

There are 7 strong acids: HCl, HBr, HI, HClO₃, HClO₄, HNO₃ and H₂SO₄. The rest of the acids are weak.

Strong bases are those hydroxides that contain cation from Groups 1 and 2 of the Periodic Table (e.g. NaOH, Ca(OH)₂). The rest of the bases are weak.

NH₃ , (CH₃)₂NH, and CH₃NH₂ are bases, despite not having OH⁻ groups, because they can accept H⁺, according to Bronsted-Lowry acid-base definition.

NH₃ + H₂O ⇄ NH₄⁺ + OH⁻

(CH₃)₂NH + H₂O ⇄ (CH₃)₂NH₂⁺ + OH⁻

CH₃NH₂ + H₂O ⇄ CH₃NH₃⁺ + OH⁻

With this information, we can classify the compounds from the list as a strong acid, strong base, weak acid, or weak base.

Answer 2

Strong acids:
HCl
H₂SO₄
HI
HBr
HNO₃
HClO₄

Strong bases:
NaOH
Ca(OH)₂
KOH
CsOH
LiOH
Ba(OH)₂

Weak acids:
HF
HCN
CH₃COOH
HNO₂
HCOOH

Weak bases:
NH₃
(CH₃)₂NH
CH₃NH₂