LAB – MOLAR MASS OF A MONOPROTIC AND DIPROTIC ACID
RESEARCH QUESTION: What is the molar mass of a monoprotic and a diprotic acid?
BACKGROUND:
Acids are molecular substances that donate protons (aka hydrogen ions) in solution. Since they ionize in solution they
are considered to be electrolytes. Strong acids ionize completely and are strong electrolytes; weak acids will only
partially ionize and are therefore weak electrolytes.
Some acids produce one proton in solution and are referred to as monoprotic acids, others can produce two or three
protons and are diprotic and triprotic acids.
A titration is an analytical procedure that can be used to determine concentration of an unknown acid. Using data from
a titration and some simple stoichiometry we can determine the molar mass of an acid.
The reactions for this procedure are:
HA (s) + NaOH (aq)  H2O (l) + NaA (aq)
H2A (s) + 2NaOH (aq)  H2O (l) + Na2A (aq)
In this equation, HA represents the monoprotic acid and H2A represents the diprotic acid.
MATERIALS:
 Ring stand
 Buret clamp
 Buret
 Funnel
 Monoprotic acid (0.225 g – 0.300 g)
 Diprotic acid (0.150 g – 0.250 g)
 Phenolphthalein
 Erlenmeyer flask
 Standardized NaOH solution
PROCEDURE:
1. Find mass of HA and cup.
2. Place HA in flask and find mass of empty cup.
3. Add 50 mL of DI water to the flask and add 3 drops phenolphthalein.
4. Fill buret with standardized NaOH solution & record initial volume.
5. Add NaOH to flask while swirling the solution until the solution remains a light shade of pink.
6. Record the final volume.
7. Repeat the procedure with a diprotic acid.
DATA:
Trial 1 (HA)
Mass of cup + acid
Mass of cup
Mass of acid
Initial volume of NaOH
Final volume of NaOH
Volume NaOH used
Trial 2 (H2A)
ANALYSIS:
1. Determine the moles of NaOH used.
2. Determine the moles of acid.
3. Determine the molar mass of each acid.
CONCLUSION:
1. Determine percent error.
2. Discuss possible source(s) of error and how the error(s) affected your molar mass.
POST LAB:
A 0.500 gram sample of a weak, nonvolatile acid, HA, was dissolved in sufficient water to make 50.0 mL of solution. The
solution was then titrated with a standard NaOH solution. Predict how the calculated molar mass of HA would be
affected (too high, too low, or not affected) by the following procedures. Be sure to explain your reasoning.
a) Extra water is added to the 0.500 gram sample.
b) An indicator that changes color at pH5 is used to signal the equivalence point.
c) An air bubble passes unnoticed through the tip of the buret during titration.