Lab 3: Percent Composition

Remind Students:

• Goggles MUST be worn whenever ANY group has chemical at their station.
• Clarify the major safety concerns
  • Acids are caustic
  • Hydrogen gas is flammable
  • Metal reacts with water
  • Etc.
• Make sure they secure all equipment in their bin when moving the bin between the cart and their stations.
• All glassware must be cleaned and dry (in their bin) before leaving lab.

When reviewing the protocol explain:

• Have them fill the water bath before doing anything else.
  • They should fill this ~ 2/3 full (filling too full will result in overflow).
• Explain how to fill the graduated cylinder completely with water and invert without having any bubbles. It may take them several tries to do this.
• If they do have an air bubble, just tell them to record the ‘initial’ volume of gas and subtract that later to find the volume of hydrogen gas produced.
• You will need to show them how to use the analytical balance at the front of the room.
  • They have to use this to have 2 significant figures in the metal’s mass.
• Do not touch metal (use scoopula).
• Tell them to have BOTH acid AND metal ready before adding anything to the flask to prevent unintended reactions (especially for Trial 2).
• They should add metal and then acid quickly and so that the chemicals go to the bottom (not along the wall) of the flask.
• They may need to press down on the stopper to prevent leaks. There is always a group who sees the reaction but who did not collect any gas. Pressing on the stopper ensures a tight seal and fixes this issue.
• Explain to them why the water level inside and outside the graduated cylinder should be as close as possible when reading the volume of gas after their trials. They have not studied pressure yet but give them an indication of why. It will not be possible for them to have the two levels equal due to the size of the bin, but they should get as close as possible.
• Make sure they thoroughly clean (and dry) the flask between trials and before they leave.