Demonstration: Law of Conservation of Mass

Purpose: In a chemical reaction, how does the total mass of the products compare to the total mass of the reactants.

Background: We will carry out this experiment in a sealed container so that if any of the products of the chemical reaction are gases, the gases will remain trapped in the flask. Your teacher will demonstrate for you the arrangement of the flask and the test tube for this experiment.

Procedure:

  1. Pour enough sodium hydroxide into a flask so that the bottom of the flask is covered with it.

CAUTION: SODIUM HYDROXIDE IS CORROSIVE. BE SURE THAT YOU ARE WEARING YOUR SAFETY GOGGLES FOR THE ENTIRE EXPERIMENT AND THAT YOU DO NOT RUB YOUR FACE IF YOU GET SOME OF THIS ON YOUR SKIN. ON THE OTHER HAND, IF YOU DO GET SOME OF THIS ON YOUR SKIN, DO NOT PANIC. WHILE IT IS CORROSIVE, YOU DEFINITELY HAVE TIME TO RINSE IT WITH WATER BEFORE ANY DAMAGE IS DONE. AT THE END OF THIS LAB, BE SURE TO WASH YOUR HANDS.

  1. Pour copper(II) sulphate into the small test tube until the test tube is about half full. In words, describe the contents of the flask and test tube in the table below.
  2. Tilt the flask to one side and carefully slide the test tube down inside the flask until it comes to rest at the bottom. Do not let the two chemicals mix. Place a stopper firmly on the mouth of the flask, sealing the flask.
  3. Determine the mass of the flask, test tube and chemicals using the electronic balance at the front of the class. Record it below.
  4. Tip the flask so that the two chemicals will mix. Find the mass of the flask apparatus now that the chemicals have been allowed to mix. Record the mass below.
  5. Remove the stopper form the flask and dispose of the chemicals in the waste container provided by your teacher.

Observations:

Mass of chemicals and glass apparatus before mixing ______

Mass of chemicals and glass apparatus after mixing ______

Description of sodium hydroxide solution before mixing
Description of the copper (II) sulphate before mixing.
Description of the mixture after being mixed.

Discussion/Conclusion: These should be answered on a separate piece of paper, in full sentences.

  1. When the solutions were mixed, did a chemical change occur? What evidence backs up your conclusion. Be specific.
  2. How did the mass of the reactants and glassware before the reaction compare with the mass after the reaction?
  3. If you noted a change in mass, explain why the mass changed.
  4. If we consider the chemical reaction in this experiment to be typical, what would you conclude about the total mass of reactants in comparison to the total mass of the products in the chemical reaction?
  5. Suppose that you performed a reaction in a flask by mixing two liquid chemicals together, but this time you did not stopper the flask. If one of the products is a gas, predict whether the mass of the products and glassware would be greater than or less than the mass of the reactants and glassware. Explain the reasoning behind your answer.
  6. State the law of conservation of mass.