Experiment

2

 

 

Volume of Liquids

Overview

There are several devices used in the laboratory for dispensing measured volumes of liquid substances and solutions. In this experiment, you investigated the 10- and 50-mL graduated cylinders, a 25-mL volumetric pipet, and a 50-mL volumetric buret. You dispensed nominal volumes of water from each of these devices, and then checked that the volumes dispensed were correct by determining the mass of the water dispensed. Mass can be determined to much greater precision than can volume and can be used to check the accuracy and precision of the volume dispensed.

Calculations

Most of the calculations in this experiment are simple subtractions. For example, if you weigh an empty beaker, and then weigh the same beaker with a sample of water in it, the mass of the water in the beaker is the difference between the two masses.

The density of water is used to compare the apparent volume of water dispensed from the graduated cylinder, pipet, or buret with the true volume based on the mass of the water. The densities of water at various temperatures is indicated in Table A-3 on Page 276.

density = mass/volume

mass = volume X density

volume = mass/density

For example, suppose you dispensed a 25-mL sample of water with the pipet, and then you weighed the water and found that its mass was 24.995 g at 25 degrees. The density of water at 25 degrees is 0.99707 g/mL (from Table A-3)

The true volume of the water would be given by

volume = mass/density = 24.995 g/0.99707 = 25.068 mL = 25.07 mL (to two decimal places)

That would mean that the pipet nominal volume is in error by  (25.07 - 25.00) = 0.07 mL.