CHM 144 Experiment 5

estimate equivalence point using indicator dye

(phenolphtalein)

adjust NaOH solution level to _____ as it's easier for calculations

0.00

If 25.31 mL of 0.0500 M NaOH solution are required to reach the equivalence point of an acid-base titration of 40.0 mL of 7-Up, how many moles of NaOH were required? Make sure your answer has the correct number of significant figures.

0.00127 moles

graduated cylinders are read to _______ decimal place

1

buret volumes are read to _____ decimal places

2

ml of colored sodas

20

for exp 2, use

250 ml beaker and not erlenmeyer flask

In the acid base reaction of aqueous citric acid with aqueous sodium hydroxide. How many moles of base are needed to react with (neutralize) one mole of citric acid?

3

keep pH electrode

3cm away from bottom of beaker

Based on your answer to the previous question, how many moles of citric acid are in the 40.0 mL of 7-Up?

4.23 x 10-4 moles (divide by 3)

ml of colorless sodas

40

Including both rapid and accurate titrations, how many trials will are conducted in Part I (Traditional Titration Using an Indicator Dye)?

5

for exp. 2 add NaOH slowly until a pH of ___ is achieved, then add VERY CAREFULLY in 0.5-1 ml increments until a pH of ____ is achieved. perform ___ more accurate trial.

6.0, 10.2, 1

For the acid-base reaction of citric acid and sodium hydroxide, the equivalence point occurs at which pH?

8.5

Explain why all of the values for moles (of NaOH and citric acid) are calculated to three significant figures.

All of the values for moles (of NaOH and citric acid) are calculated to three significant figures because the molarity of NaOH (0.0531) has three significant figures. This value was used in determining the number of moles of NaOH and citric acid, alongside the volume in liters. Since the volumes of NaOH each had 4 significant figures, when you divide the two values, the answer should contain the least number of significant figures from the original numbers (the concentration of NaOH being the least with three). Thus, the molarity has three significant figures.

Citric acid is a triprotic acid. Why is it that there is only one observable end point if there are three acidic protons?

All three acidic protons are of similar strength.

The analytical method for this experiment is the acid-base titration. Explain this method in the space below.

An acid-base titration is a type of quantitative analysis that helps determine the starting amount of an acid or a base by neutralizing it with an acid or base with a known concentration. The solution of known concentration is called the titrant (in this lab the NaOH), and the solution of unknown concentration is called the analyte (in this lab the citric acid). An acid-base reaction is used in a titration, in which a hydrogen ion is transferred from an acid to a base. For the specific reaction in this lab, citric acid is considered a triprotic acid, meaning three moles of the base are needed to neutralize one mole of the acid. The equivalence point is where the number of moles of the titrant are stoichiometrically equal to the number of moles of the analyte. This is usually estimated by measuring the end point, where an indicator dye (usually phenolphthalein) changes color to signal that the amount of titrant and analyte are equal.

Why must a colorless soda be used for the traditional titration method?

Because colorless sodas allow you to see a color change to faint pink.

Carbonated sodas (like Sprite® or 7-Up®) contain carbonic acid in addition to citric acid. Explain how it's possible (in the space below) that your reported results are only for the molarity of citric acid and do not include other acids that are present in sodas.

Before the experiment began, the carbonic acid had been removed from all of the sodas by boiling them. The carbonic acid can also be removed by allowing the soda to go flat. The other acids that are present in sodas are present in very small amounts, thus not affecting the molarity of citric acid.

key reaction

C6H8O7 (aq) + 3 OH- (aq) ⟶ 3 H2O (l) + C6H5O73- (aq)

equation for soda flattening

H2CO3 (aq) -> H2O (l) + CO2 (g)

Which soda(s) cannot be titrated using the traditional (phenolphthalein) method? Select all that apply.

Jones Green Apple Orange Crush

What is the difference between the equivalence point and the end point of a titration? Explain in the space below.

The equivalence point is where the number of moles of base added is equal (stoichiometrically) to the number of moles of acid that were originally present. Figuring out the equivalence point tends to be relatively challenging, so in lab it is often estimated using the end point. The end point is done by using an indicator dye that changes color to signal that the amount of moles of acid and base are equal. Overall, the end point is an estimate of the equivalence point.

What is the purpose of calculating and showing the first derivative along with your titration curve (see Figure 7-8 on p. 83 of the lab manual)?

The first derivative was calculated in order to determine a better measure of the volume of NaOH used to reach the estimated equivalence point. The derivative helped transform the graph so that the equivalence point became obvious (as the peak of the derivative curve). Showing the first derivative along with the titration curve helped to compare the two variables in a clearer way, displaying the volume of the equivalence point with its relative pH.

Explain your reasoning for the previous question (which method gives results that align more closely to the true equivalence point), in the space below.

The titration using the pH electrode (Part 2) gives results that align more closely to the true equivalence point because it collects actual numerical data measuring the pH of the solution. On the contrary, the equivalence point for Part 1 was estimated solely by the viewer determining when the color change was significant enough to indicate an equivalence point. Therefore, the quantitative measurements in Part 2 help align the results closer to the true equivalence point.

For the traditional titration method (Part 1), the volume of NaOH used in your molarity calculations was the total volume added (Vf-Vi). Explain why the total volume should not be used in the calculations for Part 2 (titration with the pH electrode).

The total volume cannot be used in the calculations for Part 2 because we continued adding NaOH after we passed the equivalence point to form a sigmoidal curve. Therefore, the total volume of NaOH would be higher than it should be after surpassing the equivalence point, affecting the accuracy of the pH and subsequent molarity calculations.

Explain your reasoning for the previous question (can the pH electrode titration be done using colorless sodas or only done using colored sodas?), in the space below.

Yes, it can be done because the colorless sodas still carry out the same reaction as the colored sodas, so the pH value of the citric acid solution can still be measured. The reason that titration with indicator dye cannot be done on colored sodas is because it would be hard to observe the accurate color change of the solution if the soda was also colored. Therefore, because the same reaction is occurring in all of the sodas, a pH electrode titration could still be done on the colorless sodas because the pH level still changes.

fill buret

above 0.00 mark and record initial volume

citric acid (C6H8O7)

adds tartness to sweet drinks and is present in much higher concentration than other acids because it adds flavor (except carbonic acid)

carbonic acid can be removed by

allowing the soda to go flat (it was boiled in this experiment)

unknown concentration

analyte (citric acid)

Which device is used to both deliver and measure the volume of sodium hydroxide (NaOH) to the citric acid solution?

buret

sodium hydroxide is

caustic

triprotic acid

citric acid- can donate 3 protons of all similar strength

start added in 2 ml increments while swirling

continue until faint pink persists

clean buret with

deionized water

make a _____ of the graph to measure equivalence points

derivative

an estimate of the equivalence point is the

end point

well rinse walls of buret with NaOH about 3 times with 5-10ml of NaOH to

ensure walls are well rinsed

accurate titrations

estimate the NaOH needed using rapid initial titration, performed titration 4 times and calculate rations of titrant volume to soda volume, perform a Q test.

measure meniscus at

eye level

replace waste beaker with erlenmeyer flask

filled with amount of soda, add enough deionized water to get to 100ml. quantitatively transfer solution in graduated cylinder to flask, add dye

Volume of NaOH

final-initial divided by 1000

polyprotic acid

has multiple acidic protons (citric acid)

Any titration requires that the reaction be monitored to determine when the equivalence point has been reached. For acid-base titrations and the experiment you will conduct, the most common methods rely on

measuring the pH

to determine the end point of a titration, most methods rely on

measuring the pH of the solution (pH=-log[H+])

units of molarity

mol/L

moles of NaOH

molarity (0.0531) x volume (L)

0.0531 M NaOH

molarity of titrant

moles of citric acid

moles of NaOH divided by 3

Traditional titration

must see a color change, sodas must be colorless. buret can measure volumes 2 digits beyond decimal point. keep adding drop wise until faint pink color persists. volume reading is final volume - initial reading, then do stoich

monoprotic acid

only one proton available (HCl)

titration of a weak acid (citric) is more complicated as the end points occur at

pH 8.5

pH measured on a log scale because hydrogen ion concentrations in water can range far. measured with

pH electrode or indicator dye that changes color in a pH range of 8.2-9.2

after removal of carbonic acid, while other concentrations of acids are present in small amounts, the larger amount of citric acid can be

quantitatively determined by an acid-base titration

the end point actually

slightly overestimates the equivalence point slightly

known concentration

titrant (NaOH)

In experiment 5, students will determine the concentration of citric acid in a carbonated beverage, using a solution of sodium hydroxide (NaOH) of known concentration. Using a solution of known concentration to determine the concentration of another solution through a monitored reaction of known stoichiometry that goes to completion is called a(n) ________________.

titration

objectives

to understand the difference between polyprotic and monoprotic acids to learn the basics of the pH scale to learn how to perform an acid-base titration to compare and contrast two methods for the detection of the estimated equivalence point of an acid-base titration to calculate the molarity (mol/L) of citric acid in two sodas.

an acid-base titration where a hydrogen ion is

transferred from an acid to a base

sources of error for a buret

trapped air bubbles (volume dispensed), not reading volume at eye level (parallax error), and over/under estimating the end point color (volume dispensed)

When does the equivalence point occur in an acid-base titration?

when the number of moles of acid is stoichiometrically equal to the number of moles of base added.

equivalence point

where the number of moles of base added is stoichiometrically equal to the number of moles of acid originally present in the analyte

modern titration

works in colored sodas/solutions. use magnetic stir plate and stir bar to thoroughly mix reactants. electrode monitors pH of the solution as the NaOH is added. the estimated equivalence point is the point of most rapid pH change in a plot between pH and titrant volume.