Calculate the pH of a solution for both strong acids (complete dissociation) and weak (monoprotic) acids.
* Enter the acid concentration (M) and, for weak acids, its pKa value.
Step 1: Enter Parameters
Example: 0.10 M
The pH of a solution measures its acidity or basicity and is defined as
the negative logarithm of the hydrogen‑ion concentration:
\(\text{pH} = -\log_{10}\!\bigl([\text H^+]\bigr)\).
In water, pH usually ranges from 0 (very acidic) to 14 (very basic), with 7
considered neutral. Each whole‑number change in pH represents a ten‑fold change in
\([\text H^+]\). For example, a 0.01 M HCl solution has
\([\text H^+] = 0.01\;\text M\) and
\(\text{pH} = -\log_{10}(0.01) = 2\).
Knowing pH is vital in chemistry, environmental science, biology, and industry.
This tutorial explains what pH means, the difference between strong and
weak acids, how to calculate pH in each case, and how to use the
online
pH calculator for acids
.
What Is pH and Why Is It Important?
- Definition: pH quantifies \([\text H^+]\): \(\text{pH} = -\log_{10}\!\bigl([\text H^+]\bigr)\).
- Scale & Meaning: pH 7 = neutral; pH < 7 = acidic; pH > 7 = basic. Each pH unit = 10× change in \([\text H^+]\).
- Chemical Significance: pH influences reaction rates, equilibrium positions, solubility, enzyme activity, and environmental health (e.g., acid rain, aquatic life).
- Big Idea: Lower pH → higher \([\text H^+]\) → more acidic.
Strong vs. Weak Acids
Strong Acids
Strong acids dissociate completely in water, so
\([\text H^+] \approx c\) (their molarity).
Examples: HCl, HNO3, first proton of H2SO4.
A 0.10 M HCl solution therefore has
\([\text H^+] = 0.10\;\text M\) and
\(\text{pH} = -\log_{10}(0.10) = 1\).
Weak Acids
Weak acids dissociate only partially, establishing equilibrium: \(\text{HA} \rightleftharpoons \text H^+ + \text A^-\). Their strength is measured by the acid‑dissociation constant \(K_a\) (or pKa = –log Ka). Common weak acids include acetic (pKa ≈ 4.75), formic, and citric acids.
Key Differences
- Strong acids → 100 % ionization; \([\text H^+] \approx\) initial concentration.
- Weak acids → partial ionization; need equilibrium math to find \([\text H^+]\).
- Conductivity & pH differ sharply at equal molarity.
How to Calculate pH of Strong and Weak Acids
1. Strong Acid pH
For a monoprotic strong acid, every molecule donates an H+:
\([\text H^+] = c\).
Hence
\(\text{pH} = -\log_{10}(c)\).
Example: 0.01 M HCl → pH = 2.
2. Weak Acid pH
The equilibrium expression is
Solve the quadratic for \(x = [\text H^+]\):
Then
Approximation for very weak acids: \(x \approx \sqrt{K_a c}\).
How to Use the Online pH Calculator for Acids
- Open pH Calculator for Acids .
- Enter the acid concentration (
M). - Select Strong Acid or Weak (Monoprotic) Acid.
- If Weak Acid, enter its pKa.
- Click Calculate pH to display the result and the underlying formula.
Example Problems
Example 1 — Strong Acid
Compute the pH of 0.01 M HCl.
\([\text H^+] = 0.01\;\text M\) ⇒
\(\text{pH} = -\log_{10}(0.01) = 2\).
Example 2 — Weak Acid
Find the pH of 0.10 M acetic acid (pKa = 4.75).
\(K_a = 10^{-4.75} \approx 1.78\times10^{-5}\).
Solving \(K_a = x^2/(0.10 – x)\) → \(x \approx 1.33\times10^{-3}\;\text M\).
\(\text{pH} = -\log_{10}(1.33\times10^{-3}) \approx 2.88\).
Example 3 — Strong Acid
0.20 M HNO3 → \([\text H^+] = 0.20\;\text M\); pH ≈ 0.70.
Example 4 — Weak Acid
0.050 M HF (Ka ≈ 6.8 × 10‑4) → \(x \approx 5.8\times10^{-3}\;\text M\); pH ≈ 2.23.
Frequently Asked Questions
What is pH?
pH is –log₁₀ of hydrogen‑ion concentration; pH 7 is neutral, lower values are acidic.
How do strong and weak acids differ?
Strong acids fully dissociate in water; weak acids only partially dissociate and are characterized by Ka/pKa.
How do you calculate pH for a weak acid?
Use \(K_a = x^2/(c – x)\) to solve for \(x = [\text H^+]\), then pH = –log₁₀(x). The online calculator automates this step.
Can pH be < 0 or > 14?
Yes—very concentrated acids can have pH < 0 and strong bases pH > 14, though typical lab solutions fall within 0–14.