Chemistry Lab:  Turn it Pink!  Acid Solution Stoichiometry           

Overview: In this experiment, you will be trying to get the proportions of Ca and HCl just right so that when the reaction is complete the resulting solution’s pH will be a neutral 7 (or above 7 with a small amount of Ca added).  If you have too little Ca or too much acid, the solution will remain clear and colorless.  But when you add too much calcium the solution will turn pink.  You want to get it right the first time, but there are more factors involved than just doing the math and adding the right amounts.  In the end you will not only practice your knowledge of solutions and stoichiometry, but you will also consider some of these other “factors.”

Procedure: 

1. Write a balanced chemical equation for the reaction of Ca with HCl.
2. Obtain between 50mL and 100 mL of HCl of known concentration using a graduated cylinder.
3. Place the HCl solution in an appropriately sized beaker.
4. Calculate what mass of Ca is required to react 100% with the number of moles of HCl you have in your beaker.
5. Weigh out that mass of Ca and record this mass.
6. Add two drops of phenolphthalein indicator to the acid.
7. Add the Ca metal to the acid.  Observe and gently stir.
8. When the reaction is finished, measure the pH with a piece of universal pH paper.  Record this pH.
9. If the solution is still colorless, weigh a very small piece of Ca and add that to the acid.  Stir. (Try to find a piece of Calcium that has a mass below 0.05 g.)
10. Again when the reaction is complete, measure the pH with a piece of universal pH paper.  Record the pH.
11. Repeatedly add a small amount of calcium (0.03-0.05 g) to the acid until the solution turns pink.  Then measure the pH of this solution and stop.
12. Clean up by pouring the solution down the drain.

Questions:

1. Add up the total mass of calcium you needed just before the solution turned pink.  What percentage of the total calcium mass is the “extra” you added to turn the solution pink?
2. Give two reasons why your experimental mass of calcium added didn’t agree with your theoretical mass of calcium calculated.
3. The reaction that caused the acid solution to turn pink is a different reaction than the equation that you wrote above in procedural step 1.  Write the equation of the reaction with calcium and whatever else to show this reaction.  In a couple of sentences describe what is happening when the solution turns pink.

Write-up instructions:  In your Student Laboratory Notebook record your data and observations, show your calculations and answer the questions. (As always this write-up should be in your own words, especially different from your lab partner’s.)