Standardization of naoh with khp lab report

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STANDARIZATION OF A BASE AND TITRATION OF A VINEGAR SOLUTION

ADDITIONAL READING The concepts in this experiment are also discussed in sections 3.6 AND 17.3 of Chemistry and Chemical Reactivity by Kotz, Treichel, Townsend and Treichel, and in sections 4.3b, 17.3a, and 17.3b of Mindtap General Chemistry by Vining, Young, Day, and Botch

ABSTRACT This experiment is divided into two parts. Each student is expected to perform the experiment individually.

In Part A, you will prepare a NaOH titrant solution, then standardize it (determine its exact concentration) using the acid primary standard, potassium hydrogen phthalate, KHC8H4O4, frequently abbreviated as KHP. Note KHP is not a chemical formula.

In Part B you will use your standardized NaOH solution to determine the molar concentration of vinegar (an acetic acid, CH3COOH, solution), and convert this concentration unit to a mass percent concentration unit, and finally compare your measured mass percent concentration to the value reported on the bottle.

BACKGROUND TITRATIONS One of the most useful strategies in analytical chemistry is to use a known reagent (known composition or concentration) as a standard to analyze an unknown substance. A titration is an analytical procedure in which a solution of known concentration, the standard solution, is slowly reacted with a solution of unknown concentration. The concentration of the unknown solution can be easily calculated. Titration is often used to measure the concentration of an acid or base, but it can also be used for any chemical reaction if the stoichiometry is known.

EXPERIMENTS 6 AND 7 ARE BOTH ACID BASE TITRATION EXPERIMENTS, QUITE SIMILAR TO EACH OTHER. THE REASONS FOR DOING TWO TITRATION EXPERIMENTS

A. TO GIVE STUDENTS PLENTY OF OPPORTUNITY BOTH TO PERFECT THEIR TITRATION TECHNIQUE AND TO LEARN TO DO THE CALCULATIONS;

B. TITRATION IS THE MOST IMPORTANT TECHNIQUE LEARNED IN CHEM 1033 LAB.

YOU WILL DO A PRACTICAL LAB EXAM AT THE END OF THE SEMESTER; IT WILL BE A VERY SIMILAR TITRATION.

IT IS IMPORTANT TO REALIZE THAT TITRATION IS AN ACQUIRED SKILL, REQUIRING PRACTICE. MOST STUDENTS ARE NOT PROFICIENT AT FIRST, BUT IF YOU WANT TO BECOME EXPERT AT IT, YOU WILL GET THERE WITH PRACTICE.

It is critical that there be an observable change that signals that the titration is complete. This is called the endpoint, since it signals the end of the titration, when the equivalents of titrant added just equal the equivalents of the analyte unknown. When performing an acid-base titration, we commonly use an acid-base indicator that has one color before the endpoint but changes sharply to a different color at the pH of the endpoint.

Titrations are carried out using a specialized piece of glassware called a buret, which is long tube with a dispensing valve. The buret scale has graduated marks in units of 0.01 mL or 0.02 mL. You can apply the techniques used for reading the graduated cylinder to reading a buret. The primary difference between a graduated cylinder and a buret is that the zero mark is at the top of the buret and at the bottom for a graduated cylinder.

The titrant solution is delivered through the stopcock (the dispensing valve) and bottom tip of the buret into a receiving flask. Therefore, the volume in the buret will decrease and the meniscus will drop as the solution is delivered. It is a common mistake to read the buret volume from the bottom-up. Simply read and record the number from the top down (to the right number of significant figures, and with units);. You don’t need to add or subtract your reading from 50.

In this experiment you will place the NaOH solution into a clean buret. The solution that is placed into the buret is known as the titrant. When readying the buret, over-fill the buret above the 0.00 mL mark. The stopcock is then opened to allow the solution

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Always measure the bottom of the meniscus at eye level.

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to flow out of the tip until the solution level falls just below 0.00 mL. This process ensures that the tip of the buret is filled with the solution and not with an air bubble (more on this later). It is not critical that the initial volume of the solution be exactly 0.00 mL. However, it is important that the value be between 0.00 and 1.00 mL, and that the exact value be recorded.

Once the buret is loaded with the titrant, we can then prepare the analyte solution in the Erlenmeyer flask. It is critical that an indicator be added to the analyte solution to signal the end point; otherwise, you will not know when to stop the titration and you may “over shoot” the end point.

Before the titration is started, the initial volume of titrant in the buret is read to the nearest 0.01 mL, i.e., to two decimal places; you estimate one decimal beyond the graduations. The titrant is added slowly to the analyte solution. As the addition continues, a fleeting color change may be noted (as a result of the indicator), especially when the titrant first contacts the analyte solution. It is important that the analyte solution be swirled constantly to ensure thorough mixing. Swirl carefully to avoid splashing of the analyte solution. The endpoint is not reached until the entire solution has a permanent color change (does not disappear with swirling). With practice, it is possible to determine the endpoint within a single drop of titrant solution. Once the color change is permanent, the first titration trial is complete, and the final volume can be read from the buret. The difference between the final and initial buret reading is the volume of titrant added to the flask.

The volume and concentration of the standard are used to calculate the moles of titrant added. The moles of titrant can then be stoichiometrically compared to the moles of analyte present, allowing us to calculate the concentration of the analyte solution.

Since titration is an analytical technique, multiple trials are run to ensure consistency of the results. Titrations should be run with a minimum of two trials; however, the more trials the better your results are likely to be. If, after performing a titration, you find that one set of results does not agree with the others, it should be excluded. Ideally, an additional trial should be run to confirm the results. Titration is an accurate and precise technique; if done correctly, it is possible to perform experiments with less than 1% error. It will take some practice for you to reach this level; nevertheless, you should strive to keep your errors low. Part of your grade will be based on the accuracy of your experimental results.