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Buffer pH interactive calculator

Run the Buffer pH Calculator fullscreen
Edit the MicroSim in the p5.js editor

About this MicroSim

Two coordinated panels translate the Henderson-Hasselbalch equation into visual feedback. The left panel calculates pH from the chosen buffer system, [HA], and [A-] slider values, highlighting the current ratio, pH, and a mini composition bar. The right panel works backward from a target pH, reporting the required [A-]/[HA] ratio, NaOH needed per mole of HA, and whether the request falls inside the useful pKa ± 1 range. A shared pH number line plots both the selected pKa window and the target pH marker for quick range sense-making.

How to use

  1. Pick a buffer system from the dropdown; choose Custom system to enter any pKa value.
  2. Drag the Initial [HA] (M) and [A-] (M) sliders (0.01-1.00 M) to explore how the ratio changes pH in the left panel.
  3. Use the Target pH slider (dynamic pKa ± 2 window) to design a buffer; the right panel reports the needed ratio and NaOH per 1.00 mol HA.
  4. Reference the horizontal number line to confirm whether the target pH stays inside the highlighted pKa ± 1 effective buffer range.
  5. Note the warning banner whenever the target pH sits outside the useful range, signaling limited buffering capacity.

Learning goals (Step 1)

Item Details
Subject area Chemistry - buffers
Grade band Grades 11-12 (AP Chemistry) and introductory college
Learning objective Students will apply the Henderson-Hasselbalch equation to compute buffer pH from composition and design a buffer for a target pH by determining the required [A-]/[HA] ratio.
Bloom's level Apply
Duration 12-15 minutes (demo plus guided practice)
Prerequisites Weak acid/conjugate base concepts, logarithms, Henderson-Hasselbalch form
Assessment ideas Quick-write explaining why a chosen buffer keeps pH constant, screenshot of both panels with ratio commentary, or a prompt asking which buffer best matches a target pH

Instructional design review (Step 1.5)

Question Target Decision
Single learning objective? Yes "Students will calculate or design buffer pH scenarios with Henderson-Hasselbalch."
Number of controls 1-5 4 rows (dropdown + custom pKa input, two composition sliders, one target pH slider)
Progressive disclosure Needed Warning text and NaOH guidance appear only after adjustments
Cognitive load Moderate Panels separate composition vs. design tasks with consistent typography
Accessibility Required All controls live below the drawing region with 16 px fonts and high-contrast cues

Control inventory

# Control type Label text Value format Row
1 Select + optional number input Buffer system / custom pKa Dropdown with optional decimal pKa 1
2 Slider Initial [HA] (M) 0.01-1.00 M, step 0.01 2
3 Slider [A-] (M) 0.01-1.00 M, step 0.01 3
4 Slider Target pH pKa ± 2, step 0.05 4

Layout planning (Step 2.5)

  • Number of control rows: 4
  • controlHeight = (4 x 55) + 20 = 240 px
  • drawHeight = 540 px (title + dual panels + number line)
  • canvasHeight = 540 + 240 = 780 px
  • iframeHeight = 780 + 2 = 782 px
  • sliderLeftMargin = 220 px (150 px label + 70 px value buffer)
  • margin = 24 px

Position assignments

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// Row 1 (Buffer system selector)
controlRows[0].position(margin, drawHeight + 25);
// Row 2 (Initial [HA])
controlRows[1].position(margin, drawHeight + 90);
// Row 3 ([A-])
controlRows[2].position(margin, drawHeight + 155);
// Row 4 (Target pH)
controlRows[3].position(margin, drawHeight + 220);

Label/value alignment

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labelSpan.style('width', '150px');
valueSpan.style('width', '70px');
slot.style('width', '260px'); // slider widths expand dynamically in code

Lesson plan

  1. Teacher demo (4 min): Show how equal [HA] and [A-] keep pH near pKa, then stretch the sliders to highlight ratio impacts.
  2. Buffer design task (7 min): Students pick a target pH between 5 and 10, choose an appropriate system, and record the NaOH-per-mole guidance plus warning status.
  3. Share-out (4 min): Teams compare why their chosen buffer stayed inside/outside the effective range and suggest lab applications.

References

  1. Tro, N. J. Chemistry: A Molecular Approach, 5th ed., Pearson, 2020 - Henderson-Hasselbalch derivations.
  2. Zumdahl & Zumdahl. Chemistry, 11th ed., Cengage, 2020 - Buffer preparation strategies.