Calculate The Ph Of 0.075 M Ammonia

Interactive Weak Base pH Solver

Use this premium calculator to find the pH, pOH, hydroxide concentration, and percent ionization of an aqueous ammonia solution. The default setup solves the classic chemistry problem for 0.075 M NH3 using a standard Kb value at 25 degrees Celsius.

Ammonia pH Calculator

Quick Chemistry Facts

Reaction: NH3 + H2O ⇌ NH4+ + OH-

Why pH is above 7: ammonia is a weak base, so it accepts protons from water and produces hydroxide ions.

Typical textbook answer: the pH of a 0.075 M ammonia solution is about 11.07 when Kb = 1.8 × 10^-5.

Approximation check: because only a small fraction of NH3 ionizes, the weak base shortcut works very well here.

How to calculate the pH of 0.075 M ammonia

To calculate the pH of 0.075 M ammonia, you treat ammonia, NH3, as a weak base in water. Unlike a strong base such as sodium hydroxide, ammonia does not completely ionize. Instead, it establishes an equilibrium with water:

NH3 + H2O ⇌ NH4+ + OH-

This equation tells you that ammonia reacts with water to produce ammonium ions and hydroxide ions. The key reason the solution is basic is the formation of OH-. Once you know the hydroxide concentration at equilibrium, you can calculate pOH and then pH. For a standard chemistry problem, the accepted base dissociation constant for ammonia at 25 degrees Celsius is often written as Kb = 1.8 × 10^-5.

If the initial ammonia concentration is 0.075 M, the standard setup begins with an ICE table, which stands for initial, change, and equilibrium. Let x represent the amount of ammonia that reacts with water. Then the equilibrium concentrations are:

Initial: [NH3] = 0.075, [NH4+] = 0, [OH-] = 0 Change: [NH3] = -x, [NH4+] = +x, [OH-] = +x Equilibrium: [NH3] = 0.075 – x, [NH4+] = x, [OH-] = x

Next, substitute these into the equilibrium expression for ammonia:

Kb = ([NH4+][OH-]) / [NH3] = x² / (0.075 – x)

Because ammonia is a weak base and Kb is small, x is much smaller than 0.075. That allows the common approximation:

0.075 – x ≈ 0.075

So the expression simplifies to:

1.8 × 10^-5 = x² / 0.075

Multiply both sides by 0.075:

x² = 1.35 × 10^-6

Take the square root:

x = [OH-] ≈ 0.001162 M

Now calculate pOH:

pOH = -log(0.001162) ≈ 2.93

Finally, convert pOH to pH:

pH = 14.00 – 2.93 = 11.07

So the pH of 0.075 M ammonia is approximately 11.07. That result is exactly what you should expect for a moderately concentrated weak base. The calculator above solves this using the full quadratic equation for accuracy, but it also confirms that the approximation is excellent in this case because the percent ionization is low.

Why ammonia is treated as a weak base

Ammonia is not a hydroxide salt and does not release hydroxide by complete dissociation in the way strong bases do. Instead, it acts as a Brønsted-Lowry base, accepting a proton from water. This means the extent of reaction is limited by equilibrium rather than going to completion. The practical consequence is that a 0.075 M ammonia solution has a pH much lower than a 0.075 M solution of a strong base such as NaOH.

• Strong bases ionize essentially completely.
• Weak bases ionize only partially.
• Ammonia produces enough OH- to make the solution basic, but not enough to behave like a strong base.
• The Kb value quantifies exactly how much ionization occurs.

Exact method vs approximation method

Most students first learn the approximation method because it is fast and usually accurate for weak acid and weak base problems. However, if you want the most precise answer, you solve the quadratic equation:

x² + Kb x – Kb C = 0

Here, C is the initial ammonia concentration. For C = 0.075 and Kb = 1.8 × 10^-5, the exact solution is nearly identical to the shortcut result. That is because x is only around 1.55 percent of the initial concentration, comfortably satisfying the common 5 percent rule for approximation validity.

Method	Computed [OH-]	Computed pOH	Computed pH	Comment
Approximation using x² = KbC	1.162 × 10^-3 M	2.935	11.065	Excellent textbook estimate
Quadratic exact solution	1.154 × 10^-3 M	2.938	11.062	Best numerical accuracy

The difference between the two results is only a few thousandths of a pH unit. In classroom work, either answer may be accepted depending on instructions and significant figures.

Step by step strategy you can use on any weak base problem

1. Write the balanced equilibrium equation for the weak base reacting with water.
2. Set up an ICE table with the initial concentration of the base.
3. Write the Kb expression in terms of x.
4. Use the approximation if justified, or solve the quadratic if exactness is needed.
5. Find [OH-] at equilibrium.
6. Calculate pOH using pOH = -log[OH-].
7. Convert to pH using pH = 14.00 – pOH, or use pH + pOH = pKw if Kw changes with temperature.

Comparison: ammonia vs a strong base at the same concentration

This comparison helps show why weak base equilibrium matters. If you had 0.075 M NaOH, the hydroxide concentration would be 0.075 M because NaOH dissociates almost completely. That would give a pOH of about 1.125 and a pH of about 12.875. By contrast, 0.075 M ammonia only generates roughly 0.00115 M OH- at equilibrium, giving a pH near 11.06. That is a major difference, even though the initial molarity is the same.

Solution	Initial concentration	Base type	Approximate [OH-]	Approximate pH
Ammonia, NH3	0.075 M	Weak base	1.15 × 10^-3 M	11.06 to 11.07
Sodium hydroxide, NaOH	0.075 M	Strong base	0.075 M	12.88
Ammonium hydroxide label in commerce	Varies	Ammonia in water	Equilibrium dependent	Depends on concentration

Real constants and reference values that matter

Reliable chemistry work depends on trustworthy constants. For ammonia, the Kb value commonly used in general chemistry is around 1.8 × 10^-5 at 25 degrees Celsius. The corresponding pKb is about 4.75. Since the conjugate acid is ammonium, NH4+, its pKa at 25 degrees Celsius is approximately 9.25 because pKa + pKb = 14.00 under standard conditions.

Quantity	Typical value at 25 degrees Celsius	Why it matters
Kb for NH3	1.8 × 10^-5	Determines the extent of hydroxide formation
pKb for NH3	4.75	Logarithmic form of basic strength
Kw for water	1.0 × 10^-14	Connects pH and pOH under standard conditions
pKa for NH4+	9.25	Useful in buffer and conjugate acid calculations

Common mistakes when solving the pH of ammonia

There are several repeating errors in weak base problems. The first is treating NH3 like a strong base and setting [OH-] = 0.075 M. That gives a pH much too high. The second is solving for pH directly from the ammonia concentration without first finding [OH-]. The third is forgetting that ammonia is a base, so you usually calculate pOH first and then convert to pH.

• Do not assume complete ionization.
• Do not confuse Kb with Ka.
• Do not forget the square root step in the approximation method.
• Do not subtract from 14 until after finding pOH.
• Do not ignore significant figures if your chemistry class emphasizes them.

How percent ionization helps you check your answer

Percent ionization is a useful quality check in weak acid and weak base calculations. For ammonia, the formula is:

Percent ionization = ([OH-]eq / [NH3]initial) × 100

Using the exact hydroxide concentration of about 0.001154 M and an initial concentration of 0.075 M:

Percent ionization ≈ (0.001154 / 0.075) × 100 ≈ 1.54%

Because 1.54 percent is well below 5 percent, the approximation 0.075 – x ≈ 0.075 is justified. This is why textbook problems involving ammonia at moderate concentration are often solved quickly without the quadratic formula.

Authoritative chemistry references

If you want to verify acid base constants, water chemistry relationships, or ammonia properties, these sources are useful and credible:

• U.S. Environmental Protection Agency: Ammonia information
• NIST Chemistry WebBook
• University level acid-base equilibrium resource

Final answer for the pH of 0.075 M ammonia

Using the common base dissociation constant of ammonia, Kb = 1.8 × 10^-5, and assuming standard temperature where Kw = 1.0 × 10^-14, the pH of 0.075 M ammonia is approximately 11.06 to 11.07. If your instructor wants an exact answer from the quadratic method, you will usually report about 11.06. If your class uses the weak base shortcut with reasonable significant figures, 11.07 is the standard reported value.

The most important concept is not just memorizing the answer. It is understanding why ammonia gives a basic pH, how equilibrium limits hydroxide production, and how Kb controls the extent of ionization. Once you understand those ideas, you can solve nearly any weak base pH problem with confidence.