A. Combining Calcium and Water
- Fill a 250-mL beaker ½ full of water.
- Obtain 1 small piece of calcium. Observe the appearance of calcium metal.
- Record your observations in the data section.
- Obtain a small test tube, fill with water, cover its mouth with your finger, and invert the filled test tube into the beaker. Remove your finger when the mouth of the test tube is below the water level in the beaker. The objective is to invert the test tube without having any air bubbles.
- Drop the small piece of calcium into the beaker.
- Move the test tube so that it is directly over the piece of calcium.
- Record your observations.
- Test the resulting solution with litmus paper.
- Blue litmus paper turning to red indicates the presence of H+ (acid). Red litmus turning to blue indicates the presence of OH- (base).
- Record your observations in the data section.
- Use your observations to predict the reaction between calcium and water. Balance the reaction using correct chemical formulas.
- Discard the contents of the test tube(s) as directed by your instructor, clean all glassware used thus far and place it back in the corresponding numbered tray!
B. Calibrating the Pipettes
- For the next part of the experiment it is necessary to find the number of drops per 1mL for each of your disposable pipettes.
- Obtain an empty, clean 10 mL graduated cylinder and two disposable pipettes.
- Label 1 pipette “A.” This pipette will be used ONLY with HCl for this lab.
- Label the other pipette “B.” This pipette will ONLY be used with NaOH for this lab.
- Fill each pipette with its designated solution. Close the bottles of chemicals when not in use.
- You will perform several trials with each pipette. Choose one pipette and do all trials. Before switching to calibrate the second pipette the graduated cylinder needs to be rinsed and dried.
- Hold the pipette exactly vertical over the graduated cylinder. Slowly and without letting the drops hit the side of the graduated cylinder count the drops until you have exactly 1 mL of solution in the graduated cylinder. (Make sure you are measuring from the bottom of the meniscus.)
- Repeat this process 3 times. Recording your results in the data section. Repeat with the other pipette after cleaning and drying the graduated cylinder.
- Make sure you record the molarity of both the HCl and the NaOH in your data table (use ALL the significant figures from the bottles—there should be 3 significant figures minimum).
- Use the average number of drops per mL (round to a whole number if necessary) for Part C of the procedure.
C. Stoichiometric Relationships of Calcium, HCl, and NaOH
- Obtain two test tubes (or two 50 mL beakers), and the disposable pipettes from part B.
- Make sure the test tubes and beaker are rinsed and dried.
- Label the test tubes 1 and 2 for trial 1 and trial 2 respectively.
- Make sure to use the dropper labeled “A” for the acid solution and the one labeled “B” for the base solution.
- Tare a small weigh boat on an analytical balance (must be read to 0.001).
- Weigh out between 0.05___ and 0.20___grams of calcium turnings in the weigh boat. Record the mass in the data section
- Transfer the calcium to the first test tube. Repeat for the second test tube.
- Return to your lab station with your test tubes.
- Add the 5.__M HCl dropwise to the beaker containing calcium until you have added 2mL.
- Swirl the test tube until all evidence of a reaction has ceased. In the data second, calculate the number of moles of HCl you have added.
- Add 1 drop of methyl orange to the test tube. Record the color in the data section.
- A reddish orange color indicates the presence of acid (H+).
- Yellow indicates a more neutral solution.
- You may need to put white paper under your beaker to clearly distinguish the color.
- Add 2.5__M NaOH dropwise to the test tube, counting and swirling after each drop. Record the number of drops needed to cause the color to change to yellow.
- Calculate the number of moles of NaOH required to react with the excess reactant.
- Repeat the above procedures for 1-2 additional runs (ask your instructor if there is time for a third trial).
- On the board, record the mass of Ca, the mol HCl added, and mol NaOH added. Do NOT put any additional information on the board.
- Record the class data before you leave. Make sure you calculate ALL missing data in the table. This will provide practice with stoichiometric calculations.
1. Calculate the molar mass of calcium hydroxide and calcium chloride. (Make sure you have the correct chemical formula).
2. Write the balanced chemical reaction between HCl and NaOH.
3. If the reaction between water and calcium produce a basic solution, what products must be formed. (You may need to use your textbook to evaluate this if you have not covered reactions in lecture).
4. Why is it important to calibrate each pipette?
5. When calibrating the pipettes, why should the pipettes be held vertically?
Experimental Data and Results
Combining Calcium and Water
Observations of the calcium:
Observations of the reaction between calcium and water:
Results from litmus test:
Predicted (balanced) chemical reaction between calcium and water.
Use the above to write the predicted (balanced) reaction between calcium and hydrochloric acid.
Calibration of Pipettes
|Trial||Number of Drops Pipette A||Number of Drops Pipette B|
|Average drop/mL ( ) =|
|Molarity of solution to be used||M HCl||M NaOH|
Stoichiometric Relationships of Calcium, HCl, and NaOH
Color of the solution after HCl and Ca have reacted (but before NaOH is added).
Drops of NaOH added after the 2mL of HCl were added:
_____ _____ _____
Trial 1 Trial 2 Trial 3
|mass Ca||mol Ca||Drops HCl Added||mol HCl added||mol HCl excess after Ca reaction||mol HCl used in Ca reaction||mol NaOH|
Results, Discussions and Post-Lab Questions
1. In Part C, you added HCl to Calcium. From your observations, which reactant was in excess? Explain how you know.
2. What are the mol to mol relationships below? (Hint: write the balanced equation for each to find these)
____ mol Ca : ______ mol HCl
____ mol HCl : _____ mol NaOH
3. In the space provided, attach a graph of the relationship between mol HCl used to react with Ca (NOT the total HCl added) vs mol Ca. Use the slope to evaluate the experimental relationship between mol Ca and mol HCl. Attach the graph to this report.
4. Does the slope confirm or disagree with the relationships from #2? Explain?
5. What (be specific) errors could have contributed to the fact that your slope is not exactly a whole number ratio. Consider carefully which errors would contribute to your slope (is your slope too high or too low compared to the theoretical value.