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Acids, bases and pHChemistryCollegeabout 1:43

Find the pH of a 0.15 M weak acid with Ka = 0.00072: animated lesson

A narrated, step-by-step acids, bases and ph video lesson with captions. Final answer: pH = 1.998, pOH = 12, [H⁺] = 0.0100385 M, [OH⁻] = 9.962×10^-13 M (acidic).

Video with a synthetic English voice and captions. Pick another language for subtitles in that language, or open the interactive version.

Video narrated by a synthetic voice (Piper TTS, en_US-ljspeech-high voice (LJ Speech dataset, public domain)); the steps come from our maths engine and the answer was checked independently. Download MP4 (0.6 MB) · Captions (WebVTT)

Steps on screen

  1. 1. The problem
    Worked example
    Find the pH of a 0.15 M weak acid with Ka = 0.00072.
  2. 2. Steps, part 1
    1. Ka=x2C−xK_a = \frac{x^2}{C - x}
      Set up the equilibrium (ICE table)
    2. x2+Kx−KC=0⇒x=−K+K2+4KC2=0.0100385 Mx^2 + K x - K C = 0 \Rightarrow x = \frac{-K + \sqrt{K^2 + 4KC}}{2} = 0.0100385\ \text{M}
      Solve the quadratic exactly
    3. xC×100=6.692%\frac{x}{C} \times 100 = 6.692\%
      Percent ionisation
    4. pH=−log⁡10[H+]=−log⁡10(0.0100385)=1.9983\text{pH} = -\log_{10}[\mathrm{H^+}] = -\log_{10}(0.0100385) = 1.9983
      pH
  3. 3. Steps, part 2
    1. pOH=14−pH=12.002,[OH−]=Kw[H+]=9.962×10−13 M\text{pOH} = 14 - \text{pH} = 12.002,\quad [\mathrm{OH^-}] = \frac{K_w}{[\mathrm{H^+}]} = 9.962 \times 10^{-13}\ \text{M}
      pOH and [OH⁻] (25 °C, Kw = 1.0 × 10⁻¹⁴)
  4. 4. The answer
    pH=1.998\text{pH} = 1.998

    pH = 1.9983

  5. 5. Check
    • [H⁺][OH⁻] = 1×10^-14 ≈ 1.0×10^-14.
    • The solution is acidic.

Transcript

The problem. Here is the problem: Find the pH of a 0.15 M weak acid with Ka equals 0.00072. Convert between hydrogen ion concentration, hydroxide concentration, pH and pOH in water at 25 °C.

Steps, part 1. Set up the equilibrium (ICE table). x is the equilibrium concentration of H to the power plus, which equals the amount that ionised. Solve the quadratic exactly. Percent ionisation. Above 5 percent, so the shortcut x approximately the square root of KC would be noticeably wrong; the exact quadratic is needed. pH.

Steps, part 2. pOH and [OH to the power minus ] (25 °C, Kw equals 1.0 times 10 to the power minus 14).

The answer. So the answer is pH equals 1.998, pOH equals 12, [H to the power plus ] equals 0.0100385 M, [OH to the power minus ] equals 9.962 times 10 to the power negative 13 M (acidic). pH is a logarithmic scale: each unit is a tenfold change in hydrogen ion concentration. Below 7 the solution is acidic, above 7 it is basic, at 25 °C.

Check. [H to the power plus ][OH to the power minus ] equals 1 times 10 to the power negative 14 approximately 1.0 times 10 to the power negative 14. The solution is acidic.

About acids, bases and ph

Find pH, pOH and ion concentrations of acidic and basic solutions.

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Find the pH of a 0.15 M weak acid with Ka = 0.00072 (Lesson)