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[answered] Lab 24: Titration Concepts to explore: Understand the proce


I did most of the lab but am having trouble with the calculations at the end. I attached the doc, so that my confusion would have some context to back it up. Could you please help me with the last few problems where it says to "repeat the steps"? I am not sure if I answered the question (question #4) correctly the first time.?


Thanks in advance!


Lab 24: Titration

 

Concepts to explore:

 

?

 

? Understand the process of titration. ? Use titration with a standard Na 2CO3 solution to determine whether Swee?

 

Tarts? or Smarties? candies has more moles of acid per gram of candy. Use titration with a standard Na 2CO3 solution to determine whether Swee?

 

Tarts? or Smarties? candies requires more standard base per gram of candy

 

to reach the endpoint. Introduction

 

When do you use something known to find

 

something unknown?

 

Every day we face things that are unknown to us, and every

 

day we use what we do know to determine the unknown.

 

When you wake up in the morning and see that it is cloudy,

 

you speculate that you might need an umbrella because

 

there is a possibility of rain. You do not know that it is going

 

to rain, but you do know that it is cloudy and cloudy days

 

often lead to rainy days. You use what you know to deter?

 

mine what you do not know. In the same way, in an acid/

 

base titration, we determine the unknown concentration of

 

an acid or base with a known concentration of the opposite

 

acid or base. Figure 1: Swimming pool test kits are like a small titration ex?

 

periment. Kits like this first use a phenol red indicator to meas?

 

ure the acidity of the pool water. Next, a titrant is gradually

 

added by drop; the number of drops tells you how much acid

 

or base mixture needs to be added to the pool water to

 

achieve the ideal pH. You will use candy in this lab exercise?not to eat, but to

 

determine its acid concentration. In general, to find an un?

 

known one must know how it relates to a known. Remem?

 

ber when acids and bases are combined, a neutralization

 

reaction occurs. To determine the concentration of acid in an acidic candy solution, a base of known concentration, called

 

a standard solution, is added in small quantities until complete neutralization occurs. This procedure is called an acid?base

 

titration. When neutralization occurs, the equivalence point of the titration has been reached.

 

When the standard solution is reacted with an acid or base, how do you know when the neutralization reaction is com?

 

plete? An additional substance is required for a titration ? an acid?base indicator. An indicator provides a sudden color

 

change when a certain pH is reached. The pH at which the indicator changes color, known as the end point of the titration,

 

depends on the chemical nature of the indicator. An indicator for a titration should be chosen that changes color at a pH

 

+

 

near the equivalence point. At the equivalence point, one can assume that the total number of H ions donated by the acid

 

+

 

equals the total number of H ions accepted by the base.

 

But even more information can be learned from the titrations. You can even calculate how many moles of acid there is per

 

gram of candy if the type of acid in the candy is known. To do this, the following equation is used: In the above equation: moles acid = axMxV

 

bb

 

b

 

249 Lab 24: Titration

 

a = Reaction coefficient of the acid

 

Mb = Molarity of the basic solution

 

Vb = Volume of base used in titration converted to Liters

 

b = Reaction coefficient of the base

 

To determine the reaction coefficients of the acid and base, you must first balance the reaction equation. The reaction co?

 

efficients are just the number in front of the acid or the base in the balanced equation. In this experiment, the tart candies

 

?

 

you will analyze are SweeTarts? which contain malic acid and Smarties which contain citric acid. The reactions are shown

 

below.

 

After the reaction coefficients are determined for a titration, you then calculate how many moles of acid there are in each

 

gram of candy. An example is on the next page. SweeTarts?: Malic Acid

 

(acid ?) (base ?) HO2CCH2CH(OH)CO2H + Na2CO3 (salt?) ? Na+O2- CCH2CH(OH)CO2- Na+ + CO2 + H2O Notice that for the SweeTarts, one molecule of Na 2CO3 is required for every one molecule of malic acid. This

 

is because malic acid has 2 acid groups (?CO2H). For this reaction shown above, a is 1 and b is 1. Smarties?: Citric Acid

 

(acid ?) (base ?) (salt?) 2 HOC(CO2H)(CH2CO2H)2 + 3 Na2CO3 ? HOC(CO2- Na+)(CH2CO2- Na+)2 + 3 CO2 + 3 H2O

 

For Smarties, 3 Na?CO? molecules are required for every two citric acid molecules. This is because citric acid has

 

3 acid groups (?CO2H). Here, a is 2 and b is 3. 250 Lab 24: Titration

 

Example 1: Calculating the Moles of Acid in SweeTarts?

 

If 0.455 g of SweeTarts requires 2.88 mL of 0.25 M Na 2CO3 to reach the endpoint, how many moles of acid is there in each

 

gram of the candy?

 

Answer: Since 1 molecule of malic acid requires 1 molecule of Na2CO3 , a = 1 and b = 1.

 

Next, 2.88 mL of Na2CO3 must be converted to liters of Na2CO3 . 1L

 

1000 mL Vb = 2.88 mL x = 0.00288 L Mb is given to be 0.250 mol/L Na2CO3 . Solving for moles acid, we get: Molesacid = a x Mb x Vb

 

b Molesacid/g candy = = (1) (0.250 mol/L) (0.00288 L)

 

1 0.00115 mol

 

0.455 g = 0.00115 mol = 0.00253 mol acid/g candy Less concentrated solutions are often prepared by diluting a more concentrated solution. Dilutions of molar concentra?

 

tions are made by using the equation: MV

 

11 = MV22

 

where

 

M1 = Molarity of the starting, more concentrated solution

 

V1 = Volume needed of the starting more concentrated solution

 

M2 = Molarity of the final less concentrated solution

 

V2 = Final volume wanted of the final less concentrated solution 251 Lab 24: Titration

 

In this experiment you will start with 0.500 M Na2CO3 and you will need to make 40.0 ml of 0.250 M Na2CO3. This equation

 

can be used to find out how much of the 0.500 M Na 2CO3 is needed. M 1 would be 0.500 M, V 1 is what needs to be calcu?

 

lated, M2 is 0.250 M, and V2 is 40.0 mL. The equation then becomes: MV

 

11 = MV V1 = 22 ? V1 = MV

 

22

 

M1 (0.250 M NaCO)

 

23 (40 mL)

 

0.500 M NaCO

 

23 = 20.0 mL One way to make this solution is to first carefully measure out 20.0 mL of 0.50 M Na 2CO3 into a 100.0 mL graduated cylin?

 

der. The volume is brought up to 40.0 mL using distilled water and the solution is mixed well. Notice that the volume is

 

always brought up to the total volume. This is because if you put two different liquids together, the total amount of liquid

 

you will end up with is not always the sum of the two volumes you started with. The molecules may move closer or farther

 

apart.

 

Graduated cylinders are often accurate enough for many dilutions. However, when very accurate measurements are

 

needed, chemists will use more accurate equipment such as volumetric flasks and volumetric pipettes. 252 Lab 24: Titration

 

Pre?lab Questions

 

1. What is the difference between equivalence points and end points? 2. What would happen if you forgot to put the indicator in? 3. If 2.078 g of Smarties? requires 2.11 mL of 0.250 M Na 2CO3 to reach the endpoint, how many moles of acid is

 

there in each gram of the candy? 253 Lab 24: Titration

 

Experiment: Titration of Sour Candy

 

In this experiment, you will dissolve SweeTarts? and Smarties? in water and titrate the solutions with 0.25 M Na 2CO3. The

 

end point will be determined with grape juice as the indicator. It changes color at approximately pH 8. You will then deter?

 

mine which candy requires more Na 2CO3 per gram of candy to reach the endpoint. Finally, you will calculate the moles of

 

acid per gram for each type of candy. Materials

 

Safety Equipment: Safety goggles, gloves

 

0.5 M Na2CO3 2 Erlenmeyer flasks Smarties? candies (1 pack) Scale

 

SweeTarts? candies (8 candies)

 

Stirring rod

 

Grape juice Mortar and pestle

 

Syringe Funnel

 

Stopcock Dropper (pipette)

 

2 250 mL beakers Distilled water (heated, 50?C)*

 

*You must provide Procedure

 

Part 1: Titration of Grape Juice Indicator 254 o 1. Begin heating about 300 mL of distilled water to around 50 C. 2. While the water is heating, prepare the titrant: 40.0 mL of 0.250 M Na 2CO3. Do this by first carefully measur?

 

ing out 20.0 mL of 0.500 M Na 2CO3 into a 100 mL graduated cylinder. Use distilled water to bring the total

 

volume up to 40.0 mL. Stir the solution to mix well. 3. Pour the solution into a small beaker and record 0.250 M as the concentration of Na2CO3, Mb, in Table 1. 4. Place a waste beaker below the syringe. Carefully rinse 0.5 mL of the 0.250 M Na

 

into a waste beaker. Rinse the syringe twice. 5. Remove the syringe piston and place the stopcock on the end. Set the stopcock to close (no liquid drains

 

out). Fill the syringe slightly over the 5.00 mL mark. 6. Open the stopcock and drain the syringe into a waste beaker until the bottom of the meniscus or curve

 

touches the 5.00 mL mark. Close the stopcock. HINT: Make sure there are no air bubbles trapped in the sy?

 

ringe or the stopcock. If there are, drain the syringe into the waste beaker until the bubbles are washed out,

 

then refill the syringe and repeat Step 6. 7. Record 5.00 mL as the initial syringe volume in Table 1. 8. Add 50 mL of distilled water and 5.00 mL of grape juice into an Erlenmeyer flask. Place the Erlenmeyer flask

 

directly under the syringe/stopcock. 2CO3 through the syringe Lab 24: Titration

 

9. Hold the syringe over the flask, and open the stopcock to begin to add 0.250 M Na 2CO3 solution from the

 

syringe drop by drop. 10. The moment that the solution starts to turn green, close the stopcock and swirl the flask. Set the syringe in

 

an empty beaker if needed, being careful not to lose any of the titrant.

 

11. Continue the titration by adding one drop at a time, then swirling the flask. When it looks like the color

 

change remains after swirling, stop the titration.

 

HINT: You want to add just barely enough of the 0.250 M Na 2CO3 for the color change to stay. This

 

can be hard to see. When you think you are there, look at the syringe and see exactly how much

 

you have added (to the 0.00 mL decimal place). Remember to take the syringe reading where the

 

bottom of the meniscus is seen. Add 1 more drop while looking into the flask where it falls. Be

 

sure you have your goggles on! If there is no color change where the drop falls, you are finished

 

and record the reading you took before adding the last drop. If there is a color change, swirl the

 

flask to mix and then repeat adding a drop at a time and swirling the flask until there is no color

 

change.

 

12. Record the final syringe volume in Table 1 under Grape Juice Trial 1. Record your measurement to the 0.00

 

mL decimal place.

 

13. Calculate the volume of Na2CO3 required to titrate the grape juice, and record this value in Table 1. This is

 

the difference between the initial and final syringe volumes. Maintain a precision to the 0.00 mL decimal

 

place.

 

14. Fill the syringe to 5.00 mL and repeat the procedure. Record this data under Grape Juice Trial 2.

 

15. Average the results of the total volume of Na2CO3 required for the titration, and record this value in Table 1. Part 2: Titration of Candy

 

1. Crush 8 SweeTarts candies using the mortar and pestle. Weigh between 2.5 and 3.0 g of crushed candy and

 

place into an Erlenmeyer flask. Record the exact mass in Table 2. HINT: Candies that are of a lighter color

 

work better. 2. Add about 50 mL of hot distilled water to the flask with the candies. Gently crush and stir the candy with a

 

stirring rod until the candy is completely dissolved. Allow the solution to cool. 3. Use your 10 mL graduated cylinder to add 5.00 mL of grape juice into the Erlenmeyer flask. 4. Titrate using the 0.250 M Na2CO3 in the same way as in Part 1, except look for a definite color change. HINT:

 

The color may vary depending on the original color of the solution. 5. Once a definite color change has been achieved, record the final amount of 0.250 M Na 2CO3 (to the 0.00 mL

 

place) in Table 2 under Trial 1 for the SweeTarts?. 6. Refill the syringe to exactly 5.00 mL and repeat the procedure for the SweeTarts?. Record the data in Table

 

2 under Trial 2 for the SweeTarts?. 7. Repeat this procedure for the two Smarties? trials. Using the mortar and pestle, crush approximately 15

 

?

 

Smarties , or close to 1 pack. Weigh between 2.5 and 3.0 g of powder into a flask, and titrate as before. 8. To clean up, pour solutions down the drain. 9. Calculate the volume of Na 2CO3 required for each of the trials in Part 2. This is the difference between the

 

initial and final syringe volumes. 255 Lab 24: Titration

 

Data

 

Table 1: Titration of grape juice indicator Measurement Grape Juice Trial 1 Grape Juice Trial 2 Molarity of Na2CO3

 

Initial syringe volume (mL)

 

Final syringe volume (mL)

 

Volume Na2CO3

 

required (mL)

 

Average volume Na2CO3

 

required (mL)

 

Table 2: Titration of candy Measurement Mass of candy (g)

 

Concentration of

 

Na2CO3, Mb

 

(on the bottle)

 

Initial syringe

 

volume (mL)

 

Final syringe

 

volume (mL)

 

Volume Na2CO3 solu?

 

tion required, Vb

 

(mL)

 

256 SweeTarts?

 

Trial 1 SweeTarts?

 

Trial 2 Smarties?

 

Trial 1 Smarties?

 

Trial 2 Lab 24: Titration

 

Calculations

 

1. Determine the volume of Na2CO3 used to titrate only the candy. HINT: Take the Volume of Na2CO3 used in Part

 

2 (candy and juice mixture) and subtract the volume required to titrate the grape juice alone (Part 1 average). ? SweeTarts Trial 1: ? SweeTarts Trial 2: ? Smarties Trial 1: ? Smarties Trial 2: 2. Determine the amount of Na 2CO3 used per gram of candy (mL Na2CO3/g). HINT: Divide the volumes obtained

 

above by the mass of each candy sample. ? SweeTarts Trial 1: ? SweeTarts Trial 2: ? Smarties Trial 1: ? Smarties Trial 2: 257 Lab 24: Titration

 

3. Determine the average amount of base used per gram of candy for each set of trials (mL Na2CO3/g). ? Sweet Tarts Average: ? Smarties Average: 4. Determine the moles acid per gram of candy. HINT: This is found by dividing the moles of acid found for each

 

type of candy by the mass of the candy initially weighed out and recorded in the data table. Complete in the

 

calculation in a stepwise process.

 

Moles of acid SweeTarts ? Trial 1: a. Write the balanced reaction equation. (See prelab discussion on page 250 for equation). b. Determine the mol to mol ratio of the acid to base (What are the values a and b?). 258 Lab 24: Titration

 

c. Apply the equation using your data in Table 1. (HINT: molesacid = a x Mb x Vb / b . Be sure to

 

convert mL to L.) d. Divide the number of moles of acid by the mass in grams of the candy used. Moles of acid in SweeTarts ? Trial 2: Repeat Steps a through d for Trial 2. Find the average of the two trials: Average Moles acid = 259 Lab 24: Titration

 

Moles of acid in Smarties ? Trial 1: Repeat Steps a through c for Smarties Trial 1. Moles of acid in Smarties ? Trial 2: Repeat Steps a through c for Smarties Trial 2. Find the average of the two trials: 260 Average Moles acid = Lab 24: Titration

 

Post?lab Questions

 

1. Which candy required more base per gram of candy? Is this what you expected? Explain your answer. 2. Which candy contained more moles of acid per gram of candy? Is this what you expected? Explain your an?

 

swer. 3. How would your Na 2CO3/g of candy reported change if there was a large air bubble in the stopcock tip when

 

you started your titration? 261

 


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