in a titration lab, what are some controlled variables? and how do u standardize a solution?

Answer 1

For standardization consider the following example

Standardization of HCl
Introduction
Frequently, laboratory work involves the use of a solution of precisely known concentration. Such a solution is known as a standard solution. A primary standard is a substance that is used to standardize a solution. A good primary standard must be:

(1) available in very pure form

(2) reasonably soluble

(3) stable in the pure form and in solution

(4) nonhygroscopic and easily dried

(5) a compound with molecular mass above 100

In this experiment, we will use solid Na2CO3 (sodium carbonate), a primary standard, to standardize an HCl solution by titration. The reaction is an acid-base neutralization in which carbonic acid and sodium chloride are produced. The carbonic acid decomposes to give carbon dioxide and water. The equation for the reaction is:

2HCl(aq) + Na2CO3(aq) ---> 2NaCl(aq) + H2O(l) + CO2(g)

It is important to note that the stoichiometry between HCl and Na2CO3 is a 2 to 1 ratio.

Titration consists of the careful addition of one solution from a buret (HCl) to another substance in a flask (Na2CO3) until all of the substance in the flask has reacted. The solution added from the buret is called the titrant. For every titration, there must be a way to determine when the titration reaction is complete. In acid-base titrations, this is accomplished by adding a small amount of an organic dye, called an indicator, to the solution to be titrated. If the indicator is chosen correctly, a color change, called the end point, occurs when the moles of acid equal the moles of base. Today, the indicator bromcresol green will be used. It changes from blue to green and then yellow. The proper end-point however is green. If you tirate to the yellow color, you have gone to far. When Na2CO3reacts with HCl, carbon dioxide (CO2) is produced. This will give us an incorrect endpoint. To correct this, we must remove the CO2 by boiling the solution when it first starts to turn green. The solution will turn back to blue and then can be titrated to the correct endpoint.

Procedure

A. Preparation of 0.1 M HCl

1. Dispense 4.2 mL of the 6 M HCl from a repipet into a 500 mL Erlenmeyer flask. Add 250 mL of deionized water to the Erlenmeyer. Swirl the solution and allow it to cool. Record this data in your notebook.

CAUTION: HYDROCHLORIC ACID CAN CAUSE CHEMICAL BURNS IN ADDITION TO RUINING YOUR CLOTHING. IF YOU SPILL ANY OF THE ACID ON YOU, WASH THE CONTAMINATED AREA THOROUGHLY AND REPORT THE INCIDENT TO YOUR INSTRUCTOR.

B. Preparation of the Sodium Carbonate Solution

1. Each student will need to prepare and titrate their own sodium carbonate solution. Carefully add 0.100 to 0.150 g of Na2CO3 to a clean 250/300 mL Erlenmeyer flask. Record the mass of sodium carbonate used into your notebook.
2. Add approximately 40 mL of deionized water to the Erlenmeyer flask. Add three drops of bromcresol green indicator. The solution should turn blue.

3. Use this same procedure to prepare other sodium carbonate solutions as needed.

C. Doing the Titration

Use the following format for your data table. Label the columns: Trial 1, Trial 2, and Trial 3. Label the rows: Final Volume HCl, Initial Volume HCl and Total volume.

1. Rinse a buret with at least two 3 mL portions of your HCl solution. Discard each portion into a waste beaker. Completely fill the buret with the HCl solution and remove any air in the tip by running out some of the liquid into the waste beaker. Make sure that the lower part of the meniscus is between zero and 1.00 mL on the buret. Allow the buret to stand for at least 30 seconds before reading the exact position of the meniscus. Remove any hanging drop from the buret tip by touching it to the side of the waste beaker. Record the initial buret reading. Remember, read the buret to the closest 0.01 mL.

2. Place a flask containing the sodium carbonate solution under the buret with the capillary tip inside the mouth of the flask. Place a piece of white paper under the flask. Add the HCl carefully until the solution just starts to turn green. This is the premature endpoint discussed in the introduction.

3. Place the flask on a hot plate and boil gently for a few minutes to remove the dissolved carbon dioxide. The solution should change back to the blue color.

4. Allow the solution to cool and continue the titration until the solution turns green.

5. Repeat the procedure for one more trial. If you are doing the experiment to determine the concentration of a sodium hydroxide solution, save the hydrochloric acid for next week. If not, discard the solution as directed by your instructor.


Calculations

1. Use stoichiometry to calculate the moles of HCl that were needed to titrate the sodium carbonate sample. The equation for the reaction is:
Na2CO3(aq) + 2HCl(aq) ----> 2NaCl(aq) + H2O(l) + CO2(g)

2. Using the volume of HCl needed to titrate the sodium carbonate calculate the molarity of the HCl. The two determinations should agree to within ± 0.005 M. If they do not, the titration should be repeated for a third trial.

3. Calculate the mean molarity of your three trials.


Questions

1. For each of the following errors, indicate whether the calculated molarity of HCl would be higher or lower than the real value or unaffected. Briefly explain your answer using the calculating equation for the molarity of HCl. The calculating equation for the molarity of HCl is:
M = 2(mass of Na2CO3)/(106)(Vol. of HCl)

a. The sodium carbonate was not dried before use.
b. The sodium carbonate was dissolved in 80 mL of water instead of 40 mL.

c. The buret was wet with water and wasn't rinsed with HCl before filling.

d. The solution was not boiled at the appropriate step in the procedure.


2. The green solution at the endpoint of the titration will gradually change to yellow if allowed to sit for a period of time. What would be the possible cause for this color change?

Burets
The principal use of the buret is for titrations. Precise titrations require burets that drain freely, are very clean, and do not leak around the stopcock. Teflon stopcocks can usually be prevented from leakage by tightening the tension nut, which seats the stopcock more firmly.

A clean, properly operating buret should be held in place by a buret clamp, which is attached to a ring stand. Before you fill the buret, you should rinse it several times with the solution that will eventually be in it. To rinse it, first check to make sure that the stopcock is closed. Take the buret from the clamp and use a beaker to pour about 3 to 5 mL of the solution into the buret. Carefully tip the buret on its side while holding it with your hand. Do not allow the solution to spill, but tip the buret until the solution comes in contact with almost the entire length. Rotate the buret in your hand so that the inner walls are rinsed completely with the solution. Drain the buret through the stopcock, discard this portion of the solution, and repeat the entire rinsing procedure two more times.

If you are using the buret for the first time, examine its markings before you fill it. The lines that span the entire circumference occur for each milliliter, starting with zero at the top and reaching the maximum volume of 50 ml at the bottom of the buret. As a consequence, the buret will show the volume of a liquid that has been delivered rather than the volume that remains. The smaller lines indicate each tenth of a milliliter. The spacing between these lines will allow you to estimate the volume to the nearest 0.01 mL. Thus, typical buret readings would be 9.34 mL or 17.60 mL. Readings such as 9.3 mL or 17.6 mL are not acceptable.

Fill the buret to above the zero mark with the stopcock closed. Open the stopcock fully so that the liquid drains rapidly to flush out air bubbles in the tip of the buret. Drain the buret until the meniscus rests between the zero and 1 ml marks. Do not waste time trying to align the bottom of the meniscus with the zero mark. Read the buret with your eye on the same level as the meniscus. To obtain the volume of the liquid that you use in a titration, subtract this reading from the final reading.

The left hand is used to open and close the stopcock. With a bit of practice, you will be able to adjust the stopcock so that as little as half a drop will form on the capillary tip. The right hand is used to swirl the flask. A left handed student may turn the buret 180°, then open and close the stopcock with the right hand. Unfortunately, the markings on the buret will sometimes be away from the student and will not be as easy to read.

Answer 2

You can control the volume of your solution and the amount of chemical added to solution. You can also control the volume of liquid in your titration equipment. With regard to the second question, you can buy standard solution and use these to titrate against your "stock solutions" to determine the stock solution conc.

Answer 3

controlled variable is also constant variable, the variable that will not change throughout the whole expriment. here the controlled variable is the concentration of the solution that you are gonna neutralized, the solution that is titrated, that has been prepared foreward.