Solutions

Name: ___________________

LAB: CONCENTRATIONS INTRODUCTION Solutions are made up of a solute and a solvent. There are several ways of describing how much solute is dissolved in a certain amount of solvent. Through this lab you will calculate the concentration of an unknown sodium chloride solution in molarity, molality, mole fraction and percent solution. The technique you will use to find the concentration of the unknown sodium chloride solution is called selective precipitation. You will add a small amount of a dilute potassium chromate solution to your unknown sodium chloride solution which will signal the end of the titration. The silver ions will precipitate with the chloride ions to form a white precipitate. When all of the chloride ions have been precipitated, additional silver ions will form a red precipitate with the chromate ions in solution.

PROCEDURE Standardizing the AgNO3 solution 1. Mass 0.290 grams of sodium chloride and transfer to a 50.0 ml volumetric flask. Fill the bottom of the flask with distilled water and swirl to dissolve. Fill to the 50.0 ml mark with distilled water. 2. Transfer 10.0 ml of your sodium chloride solution to a 125 ml Erlenmeyer flask. 3. Add 10 ml of distilled water to the flask. 4. Add a small amount of NaHCO3 to the flask and swirl. 5. Add 2 ml of 5% (w/w) K2CrO4 solution to the flask and swirl. 6. Titrate with the AgNO3 solution, while swirling, until the red color persists for 30 seconds. 7. Calculate the molarity of your AgNO3 solution Mass of Unknown Solution 1. Mass your unknown solution in the 100 ml volumetric flask. 2. You will mass the dry, empty flask later.

Unknown Titration 1. Transfer 10.00 ml of your unknown solution into a 125 ml Erlenmeyer flask. 2. 2. Add 10 ml of distilled water to the flask. 3. Add a small amount of NaHCO3 to the flask and swirl. 4. Add 2 ml of K2CrO4 solution to the flask and swirl. 5. Titrate with the AgNO3 solution, while swirling, until the red color persists for 30 seconds. 6. Repeat steps 1-5 for a second trial. Mass of dry, empty 100 ml volumetric flask 1. Once you have completed your unknown titration, pour out your unknown solution from the 100 ml volumetric flask and let the flask dry over night. 2. Mass the dry, empty unknown volumetric flask.

DATA You are responsible for the data tables. Make sure they are complete.

CALCULATIONS 1. 2.

Calculate the molarity and molality of your unknown sodium chloride solution. Calculate the mole fraction and percent solution (w/w) of sodium chloride and water in your unknown solution.