Name ____________________________ Partner ____________
LAB - Observing a Limiting Reactant
When two substances react, they react in exact amounts. You can determine what amounts of
the two reactants are needed to react completely with each other by using mole ratios
(stoichiometry calculations). In the laboratory, reactions are dictated by we call the limiting
reactant – the reactant that gets used up first. As soon as this reactant is used, the reaction
stops. Usually there is an excess of the other reactant – referred to as the excess reactant.
Based on quantities of both reactants and a balanced equation, you can predict which is the
limiting and which is the excess reactant.
Materials:
6 M HCL (hydrochloric acid)
Magnesium ribbon (2, 3-5 cm pieces)
Test tube
Small beaker
goggles
stiring rod
Procedure:
1. Obtain materials – 2 pieces of magnesium ribbon, test tube, beaker.
2. Determine and record the mass of your first piece of magnesium.
3. Record your observations of what the magnesium looks like (texture, color,etc).
4. Bend the piece of magnesium several times to fit in the bottom of test tube.
5. Place the test tube in the beaker and begin adding HCl. Add 10 drops and record
observations. CAUTION: do not inhale vapors or look down into the test tube.
6. After the reaction has settled, add another 10 drops of HCl. Record observations.
7. Now add HCl one drop at a time, watching to see if the reaction is still occurring. When
nothing happens, stop – your reaction is done. Be sure to record the total number of
drops you used.
8. Place the second piece of magnesium in your test tube. Record your observations.
9. Dispose of materials in waste container and clean up.
10. Answer questions and carry out calculations.
Data and Observations:
Mass of Mg __________________
Materials
Mg
Observations
Mg + 10 drops HCl
Mg + 20 drops HCl
Mg + 21 drops HCl
Mg + 22 drops HCl
Mg + _____ drops HCl
Second piece of Mg added
Questions/Analysis:
1. Write the balanced equation for the reaction between magnesium and hydrochloric acid
where magnesium chloride and hydrogen gas will be produced.
2. How many drops did you need to complete the reaction?
3. Explain why what happened, happened when you added the second piece of
magnesium?
4. Based on your observations, which was the limiting reactant? How did you know?
5. a. Calculate the number of moles in your 1st piece of magnesium.
b. If you used 1.2 moles of HCl (and your answer from above), determine how much
magnesium chloride you produced in the reaction.
c. Does your calculation match your prediction from #4?