Teacher's Pages for Mass-Mass Relationships in a Chemical Change
The setup for this lab is easy. Prepare the by diluting the stock acid with a (v:v) volume of water. Prepare enough so that each lab team has access to about .
The most common error with this lab is to add too little or too much acid. The students will often leave unreacted on the bottom of the evaporating dish because it is difficult to see the difference in color between the dish and the . Have students swirl the acid/ mixture a little and look closely, the powder will appear as a sediment.
The acid often has a way of reminding us that chemical contamination can be dangerous. If a student reports itching, wash the affected area with copious amounts of water and send the student to the nurse if warranted.
Lab – Mass-Mass Relationships in a Chemical Change
When chemical reactions occur, the amount of product created can be theoretically predicted if you know the amounts of reactant used. This phenomenon is based on the law of mass-energy conservation. Since you cannot create or destroy matter in a chemical change, the sum of the masses of reactants and the sum of the masses of products must be equal. For example:
Since this equation is balanced, the coefficients in front of each reactant are 1. One mole of must react with 1 mole of to produce 1 mole of . The object of this experiment is to prove this in the laboratory. The reaction you will be performing is as follows:
As you can see, the coefficients in this reaction are also ones. One mole of reacted with one mole of will produce one mole of , one mole of , and one mole of . You will see that the number of moles of will be equal to the number of moles of produced. The is removed by letting it escape into atmosphere, and the is removed by evaporating it in a drying oven. The mass of salt can then be determined.
The purpose of this activity is to study the relationships between the numbers of moles of reactant used, number of moles of products obtained, and coefficients used to balance a chemical reaction.
Apparatus and Materials
- Evaporating dish
- Watch glass
- Disposable pipette
- Electronic balance
- Goggles and apron
- Electric Oven
HCl is VERY strong acid! It will cause a nasty chemical burn if you get it on your skin, and will put holes in your clothing. Wear goggles and apron, and if you get it on your skin, you will begin to itch immediately. Wash it off with plenty of water.
- Weigh a clean, dry evaporating dish + watch glass on the electronic balance. Record the mass.
- Weigh out of into the evaporating dish, and record the mass.
- Measure of into a graduate.
- Place the watch glass on top of the evaporating dish, curve side down. Then add the through the remaining opening by transferring it with a disposable pipette. Add all of it. Swirl the dish slowly. Record what you see. The reaction is complete when no solids remain. Add more acid drop wise if the reaction is not complete.
- Place the dish, the liquid, and the watch glass into an oven for 24 hours.
- Record the mass of the dish, the watch glass, and the remaining residue. Record this mass.
Mass of empty dish + watch glass ___________________g
Mass of ___________________g
Mass of dish + watch glass + ___________________g
Moles of ___________________g
Moles of ___________________g
- From your balanced equation, what is the mole ratio between the and ? How does it compare with the mole ratio from your experiment?
- Suppose you had started with of , how many moles of would you expect to be formed? Explain.
- Do the results of your experiment support the law of conservation of mass-energy? Explain.