Molar Mass Determination of an Unknown Monoprotic Acid

Name

CHEM 1B Lab, Department of Chemistry, University, Fresno, CA 93740

September 11, 2018

ABSTRACT

In this experiment, acid-base titration was used to standardize a base that was then used to determine the molar mass of an unknown monoprotic acid. The base used was sodium hydroxide that was found to have a molarity of 0.4609 M, and the monoprotic acid was potassium hydrogen phthalate (KHP) with a molar mass of 207.99g per mole, with an error of 1.85%.  Thus acid-base titration was shown as an effective method of determining the molar mass of unknown acid.

Introduction

Molar mass of any substance, whether molecular or ionic, refers to the total of the atomic masses of the elements that make up that particular substance, and it is given the units grams/ mole(g/mole) (1). When the formula of the substance is known, the molar mass is calculated. In the laboratory, the molar mass of substances can be determined using volumetric analysis when the concentration of one of the reactants is known (2).

Acid-base titration can be used to determine the molar mass of an acid when using a solution of a standardized base. Monoprotic acid is one that contains one proton that will react with the hydroxide (3). A general equation for this reaction is:

HA + OH^–              H₂O   + A^–

The acid and base react in the ratio of 1:1.

Sodium hydroxide reacts with potassium hydrogen phthalate as follows:

KHC₈H₄O₄ + NaOH → KNaC8H4O4 + H2O

Potassium hydrogen phthalate acts as acid as it contains one replaceable hydrogen. From the equation, sodium hydroxide reacts with potassium hydrogen phthalate in the ratio of 1:1.

In this experiment, sodium hydroxide is standardized and then used to determine the molar mass of the unknown monoprotic acid. Table 1 shows possible monoprotic acids.

Table 1. Possible Acid Unknowns

Experimental

General Methods and Materials.

Physical Measurements.

Burets

125 ml Erlenmeyer flask

Electronic balance

Weighing paper

250 ml volumetric flask

100 ml measuring cylinder

Standardization of Sodium Hydroxide Solution.

1. The volume of the 2M NaOH solution to make 0.5 M NaOH was calculated as follows:

CIVI = C2V2

Where C1= 2 M

V1 = Unknown

C2= 0.5 M

V2 = 250 ml

2 M x V2 = 0.5M X 250 ml

2 V2 = 125 ml

V2 = 62.5 ml

2. 5 ml of the 2M NaOH solution was measured and placed into a 250 ml volumetric flask. Distilled water was then added up to the 250 ml mark.
3. A burette was filled using the 0.5 M NaOH solution prepared, and any bubbles trapped in the nozzle removed. The burette was then clamped on a ring stand.
4. 1 g of solid potassium hydrogen phthalate (KHP) was measured on an electronic balance, and the solid transferred into a 125 ml Erlenmeyer flask. The mass was recorded to 4 decimal places in table 1.
5. 25 ml of distilled water was then added to the Erlenmeyer flask and swirled to ensure the solid dissolves (exact volume not necessary as the number of moles remains constant).
6. 5 drops of phenolphthalein indicator were added to the flask, and the solution titrated against 0.5M NaOH.
7. 5 M NaOH was added slowly as the flask was swirled until a light pink color starts to linger, then it was added dropwise until when the color persists for 5 seconds. Phenolphthalein indicator is colorless in acid and pink in a base.
8. The burette reading was done, and the volume used calculated to 0.01. The volume was recorded in table 1. The volume of NaOH used, and the moles of KPH were used to calculate the molarity on NaOH solution.
9. The above procedure was repeated two more times, and table 2 completed.

Determination of Molar Mass of Unknown Monoprotic Acid.

1. A burette was filled with the 0.5 M NaOH solution and clamped on a ring stand.
2. 1 g of the unknown monoprotic acid was weighed on an electronic balance and transferred into 125 ml Erlenmeyer flask and the mass recorded to 4 decimal place in table 2.
3. 25 ml of distilled water was then added to the Erlenmeyer flask and swirled to ensure the acid dissolves. Five drops of phenolphthalein indicator were added to the solution, and the solution titrated against the NaOH.
4. 5 M NaOH was added slowly as the flask was swirled until a light pink color starts to linger, then it was added dropwise until when the color persists for 5 seconds.
5. The burette reading was made, and the volume used calculated to 0.01. The volume was recorded in table 2. The volume of NaOH used was used to calculate the molar mass of the unknown monoprotic acid.
6. The above procedure was repeated two more times and table 3 completed

Results and Discussion

The molarity of NaOH was calculated as shown:

The moles of KHP that reacted is equivalent to the number of moles present in the volume of 0.5 M NaOH used. This is because they react in the mole ratio1:1 from the equation:

KHC₈H₄O₄ + NaOH → KNaC8H4O4 + H2O

Trail 1: Given that the molar mass of KHP is 204.22g, the moles of sodium hydroxide that reacted are:

10.60 ml is the same as 0.0106 L, and it contains 0.0049 moles of NaOH.

Molarity of NaOH is:

Trial 2:

10.50 ml is the same as 0.0105 L, and it contains 0.0049 moles of NaOH.

Molarity of NaOH is:

Trial 3:

10.80 ml is the same as 0.0108 L, and it contains 0.0049 moles NaOH.

Molarity of NaOH is:

The average molarity of NaOH is:

The precision for the calculation was:

The data collected and the results of standardization of sodium hydroxide are shown in table 2.

Table 2.  NaOH Standardization and Titration Data

The sodium hydroxide was found to have exact molarity of 0.4609M. This solution was then used to determine the molar mass of the unknown monoprotic acid. The results obtained are shown in table 3.

Trail 1: The moles of monoprotic acid that reacted is equivalent to the number of moles present in the volume of 0.5 M NaOH used. This is because the monoprotic acid reacts with NaOH in the mole ratio1:1 as in the equation:

HA + OH^–              H₂O   + A^–

From the calculations above, the concentration of NaOH is found to be 0.4609 M.

1.0018 g of the unknown acid was used, these grams contain 0.004623 moles of the acid. 1 mole of the monoprotic acid will contain:

Molar mass of acid:

Trail 2:

1.0013 g of the unknown acid was used, these grams contain 0.004992 moles of the acid. 1 mole of the monoprotic acid will contain:

Molar mass of acid:

Trial 3:

1.0080 g of the unknown acid was used, these grams contain 0.004807 moles of the acid. 1 mole of the monoprotic acid will contain:

Molar mass of acid:

The average molar mass of the monoprotic acid is:

The precision for the calculation was:

Table 3.  Unknown Monoprotic acid: Titration Data and Calculations

The monoprotic acid used was thus KPH with a molar mass of 204.22 g per mole. But the experimental molar mass is 207.99 g/mol. The absolute error being:

Experimental – actual = 207.99 – 204.22

= 3.77

Thus the percentage error was:

The structure of the acid identified was:

Comments and Conclusions

Acid-base titration can be used in the determination of the molar mass of unknown acid. This is possible because if the acid is known to be monoprotic, diprotic or triprotic, it is known how many hydrogen ions will react with the hydroxide. From the stoichiometric equation, the mole ratio is used to know the moles of acid that react.

During the practical errors occur that affect the result. Some are human errors that occur during measurement of the volume of solutions and weight of solids or using more base that results in a darker pink than the expected light pink at the equivalence point.

Acknowledgment.  The author thanks the Frenso State Freshman Chemistry Laboratory staff for making it possible for the practical experiment to be a success, their guidance, and provision of materials and equipment.

REFERENCES

1. Cracolice, S. M.; Peters, E. I. Introductory Chemistry: An Active Learning Approach. Brooks /Cole Cengage Learning: Belmont, 2013; p 189-190.
2. Kotz, J.; Treichel, P.; Gabriela, W. Chemistry and Chemical Reactivity. Thomson Learning 2006; p 219-220.
3. Selvaratnam, M. A Guided Approach to Learning Chemistry, Volume 1.Juta & Co Ltd: Western Cape, 1998; p 51-53.

Supporting Information for Manuscript

The following glassware was used in this laboratory experiment.

Erlenmeyer Flask- used for titration

Wash bottle- used to add distilled water

Filter funnel- used to transfer liquids

Measuring cylinder- used to measure the volume of liquids.

Volumetric Flask- used in preparation of 0.5M NaOH from 2M NaOH

Buret- used during the titration process.