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Acid-Base Titration: A Lab Practical
Pre-lab questions
- Write the balanced equations for the reactions between NaOH and HCl and between NaOH and KHC 8 H 4 O 4 (which has one titratable hydrogen).
- How many milliliters of 0.1056 M NaOH is needed to neutralize 0.522 g of KHP? KHP is potassium hydrogen phthalate, which has the formula, KHC 8 H 4 O 4.
- A student determines that 23.88 mL of a solution of NaOH is required to neutralize 0.242 grams of KHC 8 H 4 O 4. The NaOH solution is then used in titration with HCl. If it requires 18.42 mL of NaOH solution to neutralize 20.00 mL of HCl solution, what is the molarity of the HCl solution?
- A mixture is made by combining 12.20 mL of 0.0988 M HCl with 14.35 mL of 0.106 M NaOH. Determine whether the mixture is acidic or basic; calculate the number of moles of unreacted acid or base in the solution.
Acid-Base Titration: A Lab Practical
Introduction
In this experiment, you will work with standardized solutions. A standardized solution is a solution of known molarity. Some chemicals are very pure and easy to handle. These chemicals, called primary standards, can be used directly to make a solution with a very accurate molarity. Primary standards are used to find the molarity of other solutions, called secondary standards, in a process called standardization.
The technique you will use is titration. In titration you use a solution of known molarity to find the molarity of some other solution. You can do this because the volume and molarity of the “standard” give you moles of that reactant. The stoichiometry of the reaction will give you moles of the second reactant. Molarity is moles solute per liter of solution, so you can calculate molarity if you have measured the volume of the second reactant. These are acid-base reactions, so you should be able to write the equation for the reaction that occurs with each titration. (acid + base → salt + water).
In this experiment you will make solutions of “KHP" (potassium hydrogen phthalate), a primary standard with molar mass 204.223 g/mol. KHP is a monoprotic acid; it has one titratable H. You will use your KHP solution to find the molarity of a stock solution of NaOH. Then you will use your newly standardized NaOH solution to find the molarity of HCl solutions. Your final result depends strongly on the intermediate results; hence this is a test of lab techniques you have developed this semester.
The indicator for use in this experiment has been chosen for its sharp endpoint when only a fraction of a drop of excess NaOH has been added to a solution of an acid. The indicator is phenolphthalein. It turns from colorless to a faint-pink at the endpoint and goes on to a dark pink or purple color when the endpoint is overshot. The approach to the endpoint is suggested by the temporary appearance of a pink color that fades when the solution is swirled for up to 10 seconds. A pink color that persists for more than 30 seconds signals the actual endpoint.
Suppose that you overshoot the endpoint and the solution turns bright pink. This can be corrected by back-titration. Back-titration is used when there is an excess of one of the reactants. The most common case is when one overshoots the endpoint of a titration. In back-titration, you titrate “back” to the endpoint. Drops of the initial solution are added until you are satisfied with the endpoint color of the solution. Suppose you titrate an HCl solution with some standardized NaOH. The initial solution is the HCl; the titrant is NaOH. If you overshoot the endpoint and the solution turns bright pink, you have added too much NaOH. Back-titrate to a lighter pink by adding a drop or two of the HCl. Keep track of the volumes used. To calculate molarity you will need the total volume of acid and of base used to reach the endpoint. On average, there are 20 drops per milliliter.
Chem 112 Lab Practical Summary Sheet
This must be completed and handed in before you leave the lab today.
Name: ____________________________________________________
TA: ______________________________________
Standardization of NaOH Solution Trial 1 Trial 2 Trial 3
mass of KHP
moles KHP (204.22 g/mol)
Moles NaOH
Volume NaOH used
Molarity of NaOH
Average [NaOH]
Titration of HCl solution Unknown Sample Number: _____________
Trial 1 Trial 2 Trial 3 Trial 4
Molarity of the NaOH (above)
Volume NaOH used
Moles NaOH
Moles HCl
Volume HCl used
Molarity of HCl
Average molarity of HCl
Grading: (TA use only) Student’s %error Student’s grade for this lab
