Investigating the Acid-Base Properties of Ammonium Salts

                                                                                          NAME:_________________
Another experiment courtesy of Mr. Sogo                                                       

Objective:  Apply all you know about acids & bases as you perform some experiments with various ammonium salts.

Write-up instructions:  Answer the questions posed in the space provided.  Give detailed answers on those questions that have large spaces for their answers.

Procedure:

1.  Obtain a tube containing a small sample of ammonium chloride.  Also obtain a tube containing a small sample of ammonium bicarbonate (or hydrogen carbonate).

Q1.    What are the correct formulas for the two salts mentioned above?

2.  Carefully smell the contents of the tubes.  Do not get your nose too close to the mouth of either tube.

Q2.  What gas do you suppose you are smelling?  Give name and formula.

Q3.  Which of the following needs to happen in order for ammonium ion to turn into a gas you can smell?  (Circle the correct choice.)

i.  ammonium ion needs to lose an H+ (reacts with a base)

ii.  ammonium ion needs to gain an H+ (reacts with an acid)

iii.  acid-base reactions are not important in converting ammonium ion to a gas you can smell

Q4.  How can you explain the fact that ammonium bicarbonate has a much stronger smell than ammonium chloride?  Hint: Think about the types of ions present and what types of conjugates they are.

3.  Test the gases inside each tube with pH paper.  Do this test by folding a piece of pH paper and hanging over the side of your test tube. 

Q5.  What does the pH of the gas above the ammonium bicarbonate salt indicate to you about the relative strengths of the conjugate acid, cation and conjugate base, anion that make up the salt?

Q6.  Assuming that the pH paper is coated with some weak acid molecules (HX) that are orange in color when their H’s are attached, what do you think happens to these orange molecules when the ammonia gas hits the paper?  How can this explain the color change that you observe?

Q7.  If you leave the pH paper out on your bench top for a while, you should see that it returns to its original orange color.  How do you explain this reversion to the original color?

4.  Weigh out enough ammonium chloride to make 100 ml of a 0.2 M solution.  Then add water and dissolve the salt.  I suggest you use a beaker as the container for this step.  Using a pH meter set up on a computer at the back of the room, test the pH of your solution.

Q8.  How much ammonium chloride did you weigh out?

Q9.  Show a theoretical calculation for the predicted pH for a 0.2 M solution of ammonium chloride.  Hint:  Your calculation here should involve only the ammonium ion.  This is an ICE calculation like #27 on p. 652.

Q10.  What did the pH meter read?

5.  Now make 100 ml of 0.2 M ammonium bicarbonate.  Test its pH.

Q11.  Which solution (the ammonium chloride or the ammonium bicarbonate) has a lower concentration of free H⁺ ions?  How do you explain this on a molecular level? 

Q12.  Name at least 3 chemical species (ions & molecules) that would be “swimming” or dissolved in this solution besides ammonium ions and bicarbonate ions.  Hint:  think about various acid/base equilibria that are established.

6.  Take a chunk of solid ammonium bicarbonate and put it in a 125 ml Erlenmeyer flask.  Drop  in enough 6 M HCl to make a vigorous bubbling reaction.  When the bubbling has subsided, test the gases in the flask with pH paper. 

Q13.  Write a balanced equation for the production of the bubbling gas.  Hint:  the gas is carbon dioxide. . .

Q14.  Is the bubbling produced by an acid-base reaction?  Explain.

Q15. In the bubbling flask, some HCl vapors will float up into the air. What do you think the HCl molecules do to the pH paper to make the paper turn red?  Hint:  You will have to make a hypothesis about what kind of substance (acid, base, neither, both) the red molecules on the paper are.

7.  Put a bit (about 1 gram) of ammonium chloride in a 50 ml Erlenmeyer flask.  Make a prediction as to whether you might get a more interesting reaction by adding base (NaOH), or acid (HCl).  Then perform the experiment and carefully smell the flask.  Then test the gases produced with pH paper.

Q16.  Explain your choice of either acid or base as the more interesting reactant.

Q17.  Give a chemical equation that explains what you smelled coming out of the flask.