Measure the temperature of the water in the beaker. Fill a large cylinder with water. Measure the barometric pressure or obtain a pressure reading. Look up the vapor pressure of water at the measured temperature. About 3 mL of 3 M HCl is added to a 10-mL graduated cylinder.
Water is added to fill the cylinder.
A 1-hole stopper is used to close the cylinder opening. A thumb is placed over the open hole while the cylinder is inverted and immersed under the surface of water in a 400-mL beaker nearly filled with water. The dense acid moves to the bottom of the cylinder. When it contacts the magnesium, a reaction takes place. Gas forms, and the displaced liquid is forced through the hole in the stopper.