I need the answers for post lab questions. INTRODUCTION The instructor or textbo
ID: 537185 • Letter: I
Question
I need the answers for post lab questions. INTRODUCTION The instructor or textbook presents a problem such as "What is the molecular formula of a compound with a molecular 84 mass percent of 14.3% carbon student asks, "Where did you get that value for molar mass?" The short answer to that question is the given in the problem; however, the question may go much deeper in meaning. The more exact question to ask is: how do we know what the molecular mass is for a compound? This experiment uses a method, devised by Jean-Baptiste Dumas (a French chemist who lived from 1800-1884), for approximating the molar mass, also known as molecular mass, or formula mass. The molar mass is a ratio of mass of a substance and the number of moles of the substance. With this in mind our experiment involves vaporizing a sample of volatile liquid and using its volume, temperature and pressure in the ideal gas law to determine the number of moles of gas in the remaining sample. By cooling the sample, the vapor can be condensed to a liquid. The container and liquid can be weighed to determine the mass of the liquid. Having obtained these values, it is possible to calculate the molar mass. This experiment's result should be close to the accepted correct value for the molar mass and within the limits of the procedure's ability to give accurate values. THE EXPERIMENT Caution: NO BURNERS OR OPEN FLAMES are to be used during this experiment 1. In the hood, set up a boiling water bath using a 600 mL beaker on a hotplate. Use about 300 mL of water 2. Place a piece of aluminum foil on the top of a 125 mL Erlenmeyer flask. Fold down the foil to form a narrow, tight seal around the top of the flask. Take care to fold any foil that extends down up into the narrow band. The goal is a solid cover with foil extending no more than about 5 mm down from the top of the flask. 3. Using a 600 or 800 ml beaker prepare an ice water bath. 4. Determine and record the mass of the flask with the foil seal. Use a Pasteur pipette to make a small hole in the foil cap 5. Use the Pasteur pipette to transfer 3 or more milliliters of the liquid substance through the hole and into the prepared flask.Explanation / Answer
1. By using colligative properties the molar mass can be determined, like by knowing the
depression in freezing point or elevation in boiling point of the solution.
2. To make sure that the extra liquid gets vaporized during heating, it is not weighed when covered
in flask.
3. At STP 1 mole occupies 22.4L or 22400 mL of gas
? mole occupies 152mL
Cross multiply to get the answer,
6.7857*10-3 mol = number of moles of vapor present
mol = weight / molar mass in g/mol
molar mass in g/mol = 0.470gm / 6.7857*10-3 = 69.2633 gm/mol
4. mass of vapor that occupies the flask = 90.01-89.66 = 0.35 gms
5. % error = [ |actual - experimental| / experimental ] *100 = [(88.3-76.4)/88.3]*100 = 13.47%