experiment 7 gas laws

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Experiment-7 - lab report

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Molar mass determination, an introduction to gas laws, experiment 7.

In this experiment, the molar mass of butane is determined using the ideal gas law and Dalton’s law of partial pressures.

Learning Objectives: Students will

Learn how to use the ideal gas law and Dalton’s law of partial pressures to calculate the molar mass of a gas
Practice collection of a gas using water displacement

Experiment Objectives:

Calculate the molar mass of butane
Stastically determine the accuracy and precision of the results

Introduction

Context: texas natural gas.

In 2009, Texas consumed 3 billion cubic feet of natural gas, more than any other state in the nation. 1 Natural gas is a fossil fuel composed primarily of methane, although significant amounts of ethane, propane, and butane are present. Carbon dioxide, helium, hydrogen sulfide, and nitrogen gases are found in lesser amounts. During processing, the various gases are separated to form wet natural gas and dry natural gas. Wet natural gas is composed of hydrocarbons other than methane. Dry natural gas is composed of only methane. Household natural gas is dry natural gas. Wet natural gas is further refined, yielding ethane, propane, and butane gases. Butane gas is the subject of this lab.

Typical wt. % Methane (CH 4 ) 70– Ethane (C 2 H 6 ) 5– Propane (C 3 H 8 ) and Butane (C 4 H 10 )

####### <

CO 2 , N 2 , H 2 S, etc. balance

Chemical Principles: The Ideal Gas Law and Dalton’s Law of Partial Pressures

This experiment uses two gas laws to determine the molar mass of butane: the ideal gas law and Dalton’s law of partial pressures . Gas samples are described using four variables: Pressure (P), Volume (V), moles (n), and Temperature (T). The ideal gas law combines these 4 variables, allowing the calculation of one variable if the other three are known.

The proportionality constant, R, is the universal gas law constant. The numerical value of R depends on the units used for pressure and volume. Though there are various temperature scales, for gas law calculations, the temperature scale used is always the Kelvin temperature scale. When units for pres- sure are in atms and the volume units are in liters, the value of R is 0 L atm/mol K. Measuring pressure and volume using units other than atm or L requires either the pressure and volume mea- surements to be converted to atm and liters, or the value of R is converted from atm and liters to the measurement units. For example, R = 62 torr L/mol K when measuring pressure in torr and volume in liters. Similar conversions apply to volume measurements.

The second gas law is Dalton’s law of partial pressures. Dalton’s law states that for a gas mixture, the sum of the partial pressures for each gas in the mixture equals the overall pressure.

Ptotal = PA + PB + .....

How does this apply to this experiment? During this experiment, butane gas released from a cigarette lighter will be collected by displacing water from a graduated cylinder. Before collection, the gradu- ated cylinder is filled with water and inverted in a trough of water. As butane gas is bubbled into the graduated cylinder, the collected gas forces water out of the graduated cylinder ( Figure 1 ).

©Hayden-McNeil, LLC

Figure 1. Gas collection over water.

All liquids exert a vapor pressure above the bulk liquid. The vapor pressure increases as tempera- ture increases. Water at 28°C has a higher vapor pressure than water at 20°C. Table 1 lists the vapor pressure for water around room temperature. To correct for the pressure exerted by water, the vapor pressure of water must be subtracted from the total pressure of the gas mixture. The result is pressure of the collected gas, often referred to as the dry gas.

Pdry = Pcollected – PH 2 O

You will perform multiple trials of this experiment. This will enable you to verify that your results are reproducible and also determine the precision of your data by calculating the mean and deviation of

your measurements. The mean, or average, denoted x r, is the sum of all values in a set divided by the

number of values, n .

x r= xxx 123 ++ n f+ xn

The deviation of each measurement is the difference between the measurement, x i and the mean value x r., deviation = xx 1 – r.

More useful, though, is the standard deviation, v . The standard deviation is a statistical measure of the amount of spread in a data series and can be calculated as shown below.

####### (–)(–) (–)

Through the application of the ideal gas law and Dalton’s law of partial pressures, students will learn how to calculate the molar mass of a gas. Analysis of data quality will enable the students to draw conclusions about the quality of their data.

This is one of a handful of experiments in Chemistry 112 where a percentage of your grade will be based on the quality of your results.

A volume of butane gas will be collected by water displacement. The mass of butane in the gas sample will be determined from the loss in mass of the cigarette lighter. The pressure of the collected gas will be corrected for water vapor and the moles butane gas contained calculated. The molar mass of butane is then calculated from the mass and moles of butane.

Health, Safety, and Environmental (HS&E) Items

At all times, wear appropriate personal protection equipment (PPE), especially goggles for eye protec- tion. Handle chemicals carefully. Butane from a cigarette lighter is used during this experiment and poses a fire hazard. Do not use matches, cigarette lighters, or strikers during this experiment. Perform this experiment under the benchtop fume hood.

a water trough cigarette lighter 25 mL graduated cylinder barometer top-loading balance thermometer small plastic tubing

Experimental Procedure

During this experiment, a total of four trials will be performed. The first trial you perform will be a practice run to refine your technique. The data from this practice run will be analyzed before pro- ceeding through the rest of the experiment. This will give you the opportunity to make any necessary corrections to your technique. Subsequently, three trials will be performed where the precision and accuracy of your answer will dictate a portion of your grade.

Practice run

Record the barometric pressure and temperature of the room in your lab notebook.

Record, in your lab notebook, the mass of a butane cigarette lighter on an analytical balance to four digits past the decimal point. Attach one end of the plastic tubing to the gas outlet on the lighter.

Submerge a 25 mL graduated cylinder in the water trough. Without introducing air bubbles, invert the cylinder and slip the free end of the plastic tubing into the mouth of the graduated cylinder.

Depress the red lever on the cigarette lighter to allow butane gas to flow into the graduated cylinder. Collect 20–25 mL of gas. Set the gas pressure inside the graduated cylinder equal to the pres- sure in the room by moving the graduated cylinder up and down, without introducing air into the cylinder, so that the water levels inside and outside the graduated cylinder are equal. Record the volume of gas in the graduated cylinder in your lab notebook. Remember to estimate the volume to 1/10 of the smallest division marked.

Detach the plastic tubing connected to the butane lighter. Use an analytical balance to obtain the new mass of the lighter, to four digits past the decimal point, and record the mass in your lab notebook.

Calculate the molar mass of butane as described in steps 1–4 of the data reduction and analysis section below. Remember to account for the partial pressure of water vapor.

Experimental runs

Repeat steps 2 through 5 above. Perform three new trials of the experiment.

Return the lighters to the fume hoods and the graduated cylinders to the designated area on the bench in the back of the lab. Clean your workspace before leaving lab.

Post Lab Exercises

Data reduction and analysis.

Data Reduction

Using the room pressure and the ambient temperature in the lab, look up the partial pressure of water vapor in the lab. Then, calculate the pressure of dry gas (butane) collected.

Use the ideal gas law to determine the number of moles of butane collected. Keep in mind that the units for pressure, volume and temperature need to match the units used for the universal gas constant, R, when performing this calculation.

Determine the mass of butane collected.

❏ Data sheet stapled to back of DRA form.

Experiment parameters:

Room temperature oC

Room temperature K

Barometric pressure torr

Partial pressure of water torr

Calculate the pressure of dry gas collected.

General Equation: Calculation:

Pressure of dry gas ________________

Data, Practice Run:

Practice Run Initial mass of lighter

Final mass of lighter

Mass butane used

Volume gas collected

Data Reduction and Analysis Experiment 7

Name Lab Partner

Section TA Name Score

Calculate the moles of butane collected.

Moles butane collected _____________

Calculate the molar mass of butane.

Molar mass butane ________________

Data, Experimental Runs:

Trial 1 Trial 2 Trial 3 Initial mass of lighter Final mass of lighter Mass butane used Volume gas collected Moles gas collected Molar mass butane Difference (%) from actual molar mass

Average molar mass from Trials 1–3 ________________

Identify any sources of error in your experiment. Do not just say “human error;” cite specific examples based on the design or implementation of the experiment.

Could the molar mass of the following gases accurately be determined using the procedure in this experi- ment: ammonia (NH 3 ), carbon dioxide (CO 2 ), methane (CH 4 ) and nitrogen (N 2 )? Explain why or why not.

Consider the scenario where 50 mg of copper and 10 mL of 0 M nitric acid were mixed. The reac- tion that occurs is: Cu( s ) + HNO 3 ( aq ) " Cu(NO 3 ) 2 ( aq ) + NO 2 ( g ) + H 2 O( l ). All of the gas formed from this reaction is collected over water in the same manner as the lab; the volume obtained was 20 mL. What percentage of the starting copper was consumed in this reaction? Assume room temperature (22°C) and atmospheric pressure (1 atm) for your calculations.

Multiple Choice

Course : Prin. Of Chem II Supplement (CHEM 1950)

University : augusta university.

Discover more from: Prin. Of Chem II Supplement CHEM 1950 Augusta University 97 Documents Go to course
More from: Prin. Of Chem II Supplement CHEM 1950 Augusta University 97 Documents Go to course