Question

How would the accuracy of your determined Keq change if all your volume measurements were made with graduated cylinders rather than pipets?

Answer 1

The uncertainty associated with measuring the volume of a liquid with a graduated cylinder far exceeds the uncertainty associated with a pipette. In other words, a graduated cylinder is less accurate than a pipette.

A 10-mL Class A pipette has a measuring uncertainty of ±0.03 mL while a 25-mL Class A pipette has a measuring uncertainty of ±0.05 mL. In contrast, measuring the same volume of a liquid with a graduated cylinder invokes a much higher uncertainty. A 10-mL graduated cylinder has an uncertainty of ±0.20 mL; similarly, a 25-mL graduated cylinder has an uncertainty of ±0.35 mL. Thus, it is clearly understood that the volume measured by a graduated cylinder is less accurate than the same volume measured with a pipette.

The K_eq of a reaction is defined as the ratio of the molar concentration(s) of the products divided by the molar concentration(s) of the reactants. The molar concentration of a species is defined as the number of moles of the species contained in unit volume of the solution made with the same species. When the measurements are made with a graduated cylinder instead of a pipette, the uncertainty associated with the volume(s) of the reactants and the products increases and thus, the K_eq becomes less accurate.