Pre-Lab ? (Answer on Your Own Piece of Paper)

Name______

Block______

Classic Titration

Safety

Dilute hydrochloric acid and sodium hydroxide solutions are irritating to skin and eyes. Notify your teacher and clean up all spills immediately with large amounts of water. Phenolphthalein is an alcohol-based solution and is flammable. It is moderately toxic by ingestion. Keep away from flames and other ignition sources. Avoid contact of all chemicals with eyes and skin and wash hands thoroughly with soap and water before leaving the laboratory. Wear chemical splash goggles and chemical-resistant gloves and apron.

Introduction

One of the most common questions chemists have to answer is how much of something is present in a sample or a product. If the product contains an acid or base, these questions are usually answered by titration. Acid-base titrations can be used to measure the concentration of an acid or base in solution and to calculate the formula (molar) mass of an unknown acid or base.

Pre-lab ? (Answer on your own piece of paper)

The appropriate concentration of hydrochloric acid is 0.5M. The exact concentration of this solution is to be determined by titration with 0.215M sodium hydroxide solution.

A 10.00mL sample of HCl solution was transferred by pipet to an Erlenmeyer flask and then diluted by adding about 40mL of distilled water. What is the approximate H3O+ concentration and pH of the solution in the flask before the titration begins?
Phenolphthalein indicator was added, and the solution in the flask was titrated with 0.215M NaOH until the indicator just turned pink (pH 8-9). The exact volume of NaOH required was 22.75mL. Calculate the concentration of HCl in the original 10.00mL sample.
One student accidentally “overshot” the endpoint and added 23.90mL of 0.215M NaOH. Is the calculated concentration of HCl likely to be too high or too low as a result of this error? Why?

Procedure

Obtain about 75mL of standard solution hydroxide solution and record the precise molarity of the solution in your data table.
Rinse a clean 50mL buret with two 5-mL portions of the sodium hydroxide solution.
Clamp the buret to a ring stand and place a waste beaker under the buret. Fill the buret to above the zero mark with sodium hydroxide solution. Open the stopcock to allow any air bubbles to escape from the tip. Close the stopcock when the liquid level in the buret is between the 0 and 10mL mark.
Record the precise level (initial volume) of the solution in the buret in your data table.
Using a pipet or graduated cylinder, transfer 10.00mL of the unknown hydrochloric acid solution to a 125mL Erlenmeyer flask. Record your unknown here______.
Add about 20mL of distilled water to the flask, followed by 2 drops of phenolphthalein indicator.
Position the flask under the buret so that the tip of the buret is inside the mouth of the flask. Place a piece of white paper under the flask to make it easier to detect the color change of the indicator at the endpoint.
Open the stopcock to allow approximately 5-8mL of the sodium hydroxide solution to flow into the flask while continuously swirling the flask. Observe the color changes occurring.
Continue to add sodium hydroxide slowly, drop-by-drop, while swirling the flask. Use a wash bottle to rinse the sides of the flask with distilled water during the titration.
When a faint pink color appears and persists for 10 seconds or more while swirling the flask, the endpoint has been reached. Close the stopcock and record the final buret reading for Trial 1 in your data table.
Pour the solution out of the flask into the sink and rinse the flask with distilled water.
Repeat the titration (steps 4-11) with a second sample of hydrochloric acid. Record all data for Trial 2 in your data table.
If time permits, complete a third trial.

Data Table

Post Lab ? (Answer and show all calculations on your own paper)

Calculate the unknown molarity of the hydrochloric acid solution for Trials 1, 2, and 3.
Find the average molarity for all three trials and record your final answer here ______.

Lab Report

Do all sections of the lab report that are on the How to Write a Lab Report sheet except for a graph because that does not apply to this lab. I will pick and choose which sections I am grading, and you will be graded on the accuracy of the molarity of the acid.