Experiment 12

Solution Stoichiometry

Learning Objectives:

Upon completion of this experiment, students will:

1. 1. (CLO3). Analyze evidence to decide if generalizations or conclusions based on the obtained data are warranted
  2. (CLO4). Interpret and utilize mathematical formulas while solving problems
  3. (MLO) Calculate the molarity of a solution using molar mass and the mole concept: convert between mass, moles, and molarity.
  4. (MLO) Determine the concentration of a solution that has been diluted in addition to applying dilution principles toward serial dilutions.
  5. (MLO) Apply concepts of stoichiometry toward reactions in solution and their associated problems using titrations and a standardized solution to determine the concentration of a common household solution.
  6. Use an indicator and a primary standard to titrate a solution and determine its molarity

Experiment 12 discussion and videos will show you real titrations and calculations of an acid base titration.

The assignments that must be completed are:

Complete the pre laboratory problems before coming to the class meeting.
Complete the laboratory report using the directions, class data and resources.
- Part A: complete three trials for the standardization of the NaOH solution. Complete all calculations and questions in the report.
- Part B: Complete three trials for the titration of the vinegar solution. Complete all calculations and questions in the report.

Experiment12

Discussion

Volumetric glassware which includes pipets, flasks and burets. The buret is a long tube that is calibrated with small markings which can be used to determine volumes with both precision and accuracy. This is the basis of volume stoichiometry. Most volumetric glassware is precise, being able to measure volumes to less than a 1% error or to + 0.01 mL. With the ability to measure volume delivered into a mixture and mass of a substance with great precision, we are able to use stoichiometry to determine the concentrations of solutions by knowing the mole ratio of the reactants or to determine the number of moles by using the concentration of the solution. The mass of the solid being titrated allows us to calculate the moles of the reactant.

O-Experiment12 Discussion

moles of solute (KHP) = mass of solute / molar mass

The balanced reaction gives us the mole ratio of the two reactants, allowing for the determination of the moles of the other reactant. But how do we know when the two often colorless compounds when dissolved in water are in equivalent amounts, called the equivalence point.

NaOH + KC8H5O4 (KHP) ⇄ KNaP + H2O

Note KHP has one acidic hydrogen and NaOH has one equivalence of base (hydroxide) therefore this is a 1 to 1 molar ratio.

Since we are using acids and bases in this experiment, we can use indicators which are substances that change color when the pH (acid content of the solution) changes. A solution that has a pH of 7 is said to have a neutral pH. Low pH, below pH 7, has a higher concentration of acid while pH above 7, high pH, has a higher concentration of base. If we begin this titration with an acid solution and slowly add a base, at the point when the solution is pH 7, the amount of base and acid are generally equal. This is not completely true, but if we use this concept, then by adding one more drop of base, the solution becomes slightly basic and an indicator that is sensitive to pH changes will change color. Phenolphthalein is an compound that changes in a solution that is slightly basic. We can use this change to "see" the end point of the titration. The acid solution is colorless, but as the base is added, we will begin to see the "pink" color of the indicator. When is color persists, then we have reached the end point. Since a very small volume of the NaOH caused the color change (pH greater than 7) we can estimate with some precision the equivalence point as being equal to the end point. By measuring the volume of the base added and calculating the moles of the base from the mass and moles of acid titrated, the concentration of the base can be calculated. The most common means of measuring the concentration of a solution is MOLARITY. Molarity, M, is defined as the moles of a solute, n, (the substance that is dissolved in the solvent) divided by the volume of the total solution, V, in Liters.

The balanced reaction gives us the mole ratio of the two reactants, allowing for the determination of the moles of the other reactant. But how do we know when the two often colorless compounds when dissolved in water are in equivalent amounts, called the equivalence point.

NaOH + KC8H5O4 (KHP) ⇄ KNaP + H2O

Note KHP has one acidic hydrogen and NaOH has one equivalence of base (hydroxide) therefore this is a 1 to 1 molar ratio.

Since we are using acids and bases in this experiment, we can use indicators which are substances that change color when the pH (acid content of the solution) changes. A solution that has a pH of 7 is said to have a neutral pH. Low pH, below pH 7, has a higher concentration of acid while pH above 7, high pH, has a higher concentration of base. If we begin this titration with an acid solution and slowly add a base, at the point when the solution is pH 7, the amount of base and acid are generally equal. This is not completely true, but if we use this concept, then by adding one more drop of base, the solution becomes slightly basic and an indicator that is sensitive to pH changes will change color. Phenolphthalein is an compound that changes in a solution that is slightly basic. We can use this change to "see" the end point of the titration. The acid solution is colorless, but as the base is added, we will begin to see the "pink" color of the indicator. When is color persists, then we have reached the end point. Since a very small volume of the NaOH caused the color change (pH greater than 7) we can estimate with some precision the equivalence point as being equal to the end point. By measuring the volume of the base added and calculating the moles of the base from the mass and moles of acid titrated, the concentration of the base can be calculated. The most common means of measuring the concentration of a solution is MOLARITY. Molarity, M, is defined as the moles of a solute, n, (the substance that is dissolved in the solvent) divided by the volume of the total solution, V, in Liters.

Molarity = moles of the solute / Volume in Liters of the olution

The solution that is prepared with a solute that is 99.99% pure and does not absorb water (nonhygroscopic) is called a standard solution. This standard solution can be used to determine the concentration of a second solution. The solutes used to make these solutions are called primary standards. Potassium Hydrogen Phthalate, simply called KHP, is a primary standard and is commonly used as the acid in a reaction with NaOH. If the exact mass of a solute cannot be determined because of is purity or other factors, then the primary standard is often used to titrate the solution to determine its concentration. Sodium hydroxide is so hygroscopic that its mass increases as you attempt to measure it as water is absorbed from the air.

Once the concentration of the NaOH is determined, we can use the standardized solution to determine the concentration of any acidic solution. Vinegar is a common household solution that contains acetic acid as the solute. Most vinegar solution are between 5% and 10% by mass. We will use the calculated molarity of the NaOH solution and the volume of NaOH used to reach the end point of the titration beween acetic acid and NaOH to determine the molarity of the vinegar solution of acetic acid. The moles of NaOH can be calculated :

moles of solute = Molarity of solution x volume of solution ( n = M V)

By knowing the molar ratio of the reaction between acetic acid and NaOH, we can determine the moles of acetic acid titrated.

HC2H3O2 + NaOH ⇄ NaC2H3O2 + HOH

The molarity of the vinegar solution will be moles of acetic acid calculated divided by the volume of the vinegar solution. However, most household solutions are not reported as molar concentration but as mass percents. The mass percent is determined from the mass of acetic acid over the mass of the vinegar solution (times 100%) The mass of acetic acid is determined from the moles above and the mass of the vinegar solution is measured before the titration begins.

There are many errors that can be made in this experiment but by being careful of contamination and poor techniques, this is a very precise technique. The following videos explain the steps that need to be followed to preform a titration with precision.

https://youtu.be/zOcduvKFh3U
UNL Chemistry
Duration: 8 min 51 sec.

https://youtu.be/sFpFCPTDv2w
Carolina Biological
Duration 6 min 52 sec.

Pre Writing assignment

Your instructor may require a prewriting of this experiment.

Pre laboratory assignment

The pre laboratory problems are questions about the laboratory assignment, background, definitions or procedure and calculations.

Using the Virtual Laboratory simulation at http://chemcollective.org/vlab/101 and the following procedure, practice the titration of NaOH using a KHP solution and phenolphthalein. Report the molarity of the NaOH solution.

O-Experiment12 pre lab

Lab 12 Part A simulation.mp4

This video shows the simulation of the titration of a KHP solution with a NaOH solution and the calculations needed to determine the molarity of the NaOH solution.

Procedure and Report

The laboratory report of the assignment contains questions, data collection, calculation and reporting of results.

Part A: Standardization of sodium hydroxide solution

Using a prepared NaOH solution, KHP will be measured and titrated with the prepared NaOH solution to determine the concentration.

Experiment12 Procedure

Lab 12 introduction - indicators.mp4

This video introduces Experiment 12 with the preparation of the NaOH solution and how the titration will be performed with an indicator.

KHP titration.mp4

This video shows the titration of a mass of KHP with a 0.2 M solution of NaOH using phenolphthalein indicator. Calculations of how to determine the molarity of the NaOH solution are explained.

O-Experiment12 report

Part B: Analysis of Vinegar

Three samples of a white vinegar will be titrated with the standardized NaOH from part A. The molarity and % mass will be calculated.

Class Data will be collected and will be used to complete the assignment.

vinegar titration.mp4

This video shows the titration of a white vinegar solution with the standardized NaOH solution from Part A. Calculation of how to determine the molarity and % mass of the vinegar solution are explained. The simulation is briefly mentioned.