CHEMISTRY

EXPERIMENT 14--How Bonding Affects Acidity

Introduction:

In many of the common acids you have used in laboratory work, all hydrogens are acidic. This is true for acids such as sulfuric acid, nitric acid, and hydrochloric acid. In this laboratory activity, you will learn that within selected acid molecules only some hydrogens may be capable of forming H+.

In this experiment you are going to experimentally determine the number of ionizable H+ in an unknown solid acid and to form hypotheses regarding the relation(s) between bonding and acidity.

The basic definition of an acid is that it is a substance which produces hydrogen ions in water. When hydroxide ion is added to an acid, (this is the technique of titration) the two ions come together to form water, one hydroxide ion for each hydrogen ion. This is a type of ionic reaction, and is specifically called a neutralization reaction, because if all the hydrogen ion is reacted with an equivalent number of hydroxide ions, the solution will be neutral. The exact point as which every hydrogen ion has been neutralized with an hydroxide ion is called the equivalence point. By knowing the concentration of the hydroxide ion and the amount of base used to titrate a known mass of acid, one can calculate the concentration of acid, and thus the amount of hydrogen ion. The net ionic reaction for this is

H⁺_(aq) + OH^-_(aq) ---> H₂O_(l)

Procedure:

Select one of the un-named acid samples and record its molecular formula.

Obtain a buret, and if not suitably clean, wash with fresh, warm, soapy water and rinse well. Then rinse with distilled water, followed with a few mL of the 0.250 M sodium hydroxide sol'n to be used in the experiment. Make sure the stopcock does not leak, and then fill the buret with the 0.250 M NaOH. Be sure to record this and all buret readings to the nearest .01 mL.

Add between 0.50 and 0.80 g of the acid to a 250 mL Erlenmeyer flask and dissolve in 50.0 mL of distilled water. Add 3 drops of phenolphthalein.

Slowly add the sodium hydroxide to the acid solution, swirling the flask constantly until a faint pink color persists for at least 30 seconds. (Be sure there is no undissolved acid) (A sheet of white notebook paper will aid in seeing the pink color) Do not overshoot the equivalence point. If you add too much base, mass out a little more of the solid acid and add to the flask, dissolve, and continue the titration.

Repeat the titration with two more samples of the un-named solid acid.

Data Analysis:

First, for each titration, calculate:

Second, determine the average ratio of moles of hydrogen ions to moles of acid. This value will give the number of ionizable hydrogens in each acid molecule. Rewrite the molecular formula from the vial to reflect the number of ionizable hydrogens. Draw a Lewis structure consistent with the molecular formula for your acid.

Develop an hypothesis to account for your experimental results.

Applications:

Answer the following questions using the structural formulas given below them.

1. Which pair are isomers?

2. Which contain two ionizable hydrogens per molecule?

3. Which contain one ionizable hydrogen per molecule?

4. How many of the compounds are acids?

5. Which acid is capable of losing three ionizable hydrogens per molecule?

6. Which acid has a molar mass of 60.0 g/mol?

7. Which species contain OH groups that do not have ionizable hydrogens?

8. Which pair of acids contain two carbon atoms per molecule?

9. How many ionizable hydrogen are there in i?

10. How many ionizable hydrogen are there in f?

Structural formulas are on their way.

Gwen Sibert
RoanokeValley Governor's School
gsibert@RVGS.k12.va.us