Lab Skills

Liquid Displacement

Before-and-after beakers show water rising as a steel cube submerges. ΔV gives the object's volume — works for any irregular shape.

ΔVVolume of SolidBeakerIrregular Shapes
Topic L.7

Experimental Design and Error Analysis

Identify a testable scientific question based on an observation, data, or a model; explain how modifications to an experimental procedure will alter results.

The AP Chemistry free-response section reliably includes an experimental-design question. The CED science practices behind them are 2.A (identify a testable question), 2.C (identify aligned procedures), 2.E (identify sources of error), and 2.F (explain how modifications alter results).

What a full-credit procedure contains:

  • Specific glassware and instruments, with the precision implied. "Use a 25.00 mL volumetric pipet", not "measure out some solution".
  • The quantity to be measured and how it will be measured.
  • A controlled variable list — what is held constant so the comparison is valid.
  • The calculation that turns the measurement into the answer.

Answering "too high or too low" questions. Work forward mechanically:

  1. Which measured quantity does the error change?
  2. Does it make that measurement larger or smaller?
  3. Where does that quantity sit in the final calculation — numerator or denominator?
  4. Therefore the reported value is too high or too low.

Do not guess the direction. Trace it.

Random vs. systematic, again. If the question asks how to improve the result: more trials and averaging address random error; identifying and eliminating the cause addresses systematic error. Recommending "repeat the trial" for a systematic error does not earn the point.

Justifying a claim with data (practice 6.B): cite the specific numbers, state the trend, and connect it to a chemical principle. All three parts are usually required.

Key points

  • Name specific glassware and the precision it provides.
  • Trace an error through the calculation rather than guessing its direction.
  • More trials fix random error only; systematic error requires fixing the cause.
  • A justification needs data, a trend, and a chemical principle.

Common mistakes

  • Vague procedures. "Measure the volume" earns nothing without an instrument and a precision.
  • Guessing the direction of an error.
  • Recommending more trials for a systematic error.
  • Restating the data without interpreting it.

Worked example

A student determines the molar mass of an unknown solid acid by titrating a weighed sample with standardized NaOH. For each error, state whether the calculated molar mass would be too high, too low, or unaffected, and justify: (a) the buret was rinsed with water but not with NaOH; (b) some of the solid acid was left in the weighing boat; (c) extra distilled water was added to the titration flask.

The governing calculation: M = mass of acid ÷ moles of acid, where moles of acid come from moles of NaOH delivered. Mass is in the numerator; moles in the denominator.

(a) Buret rinsed with water — molar mass too LOW.
Residual water dilutes the NaOH, so a larger volume is required to reach the endpoint. The student calculates using the nominal concentration and that inflated volume, overstating the moles of NaOH and therefore the moles of acid. A larger denominator with an unchanged numerator gives a smaller molar mass.

(b) Acid left in the weighing boat — molar mass too HIGH.
The recorded mass includes acid that never entered the flask, so the numerator is too large. Meanwhile the titration only reflects the acid actually present, so the moles in the denominator are correct for the smaller true sample. A too-large numerator over a correct denominator gives a molar mass that is too high.

(c) Extra distilled water in the flask — UNAFFECTED.
Adding water changes the concentration of the acid in the flask but not the number of moles present. The same volume of NaOH is still required to reach the equivalence point, so both the mass and the moles are unchanged and the calculated molar mass is correct. (This is a favorite distractor precisely because it looks like it should matter.)

Full notes for topic L.7 →