How to Make Use of a Titration or pH Curve to Solve Problems – Part 1

Titrations or quantitative neutralization reactions between acids and bases are often recorded on graphs called titration curves or pH curves, which generally contain the volume of the titrant (or titrator that titrates an analyte or titrand) as the independent variable (X-axis) and the pH of the solution as the dependent variable (Y-Axis).


An acid–base reaction can be used to determine pH via titration.

pH is a logarithmic scale used to specify the acidity or basicity of aqueous solutions.

Acidic solutions (with higher concentrations of hydrogen (H+) cations) are measured to have lower pH values than basic or alkaline solutions. The neutral pH is 7. A 𝑝𝐻 meter may be used for a precise and accurate measurement of the 𝑝𝐻 of a solution.

The equivalence point, or stoichiometric point, of a chemical reaction is the point at which chemically equivalent quantities of reactants have been mixed. The endpoint refers to the point at which the indicator changes color in a colorimetric titration. The endpoint is related to the equivalence point, but these two are not necessarily the same. On the pH curve, it is the inflection point.

A buffer solution is a solution where the pH does not change significantly on dilution or if an acid or a base is added at constant temperature.

Question:



Click/tap to enlarge the image

The above graph shows a pH curve of a weak base titrated with a strong acid. The pH was measured after small volumes of 0.075 M HCl were added to 25.0 mL of a weak base.

  1. If a pH indicator was used that changes color from pH 5 to pH 7, would the observed color change be distinct and the calculated molarity of the base be accurate?
  2. If a pH indicator was used that changes color from pH 6 to pH 8, would the observed color change be distinct and the calculated molarity of the base be accurate?
  3. What is the change in pH from the first point to the second on the curve that indicate the region where the solution can be described as a buffer?
  4. What value of pKI of a visual indicator will optimize the results?
  5. Will the concentration of the base be more than, equal to or less than 0.075 M?
  6. If small volumes of 0.200 M HNO3 were added to 50.0 mL of a weak base, would the concentration of the base be more than, equal to or less than 0.200 M or 0.100 M?
  7. Which arrow points to the place where the slope of the curve is the smallest?

Solution:



Click/tap to enlarge the image

Mb = Molarity of the base

Tumultuous titrations❓ Let Chemaficionado know in the comments below or at mychemistryhomework@gmail.com❗

References:

(1)    Equivalence point. Wikipedia. https://en.wikipedia.org/wiki/Equivalence_point.

(2)    Wikipedia Contributors. Buffer solution. Wikipedia. https://en.wikipedia.org/wiki/Buffer_solution.

(3)    Wikipedia Contributors. pH. Wikipedia. https://en.wikipedia.org/wiki/PH.

‌(4)    Jespersen, N. D.; Barron's Educational Series, Inc. Barron’s AP Chemistry; Barron’s Educational Series: Hauppauge, N.Y., 2012.

‌(5)    Brown, C.; Ford, M. Higher Level Chemistry, 2nd ed.; Pearson Education: Harlow, Essex, 2014.

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