Introduction
As the world of chemistry constantly evolves, certain elements, compounds, and even substances are always undergoing substance identification. Moreover, there are many methods to identify a substance. As noted by Austin Peay State University’s department of chemistry, the identification of a substance can be determined by making simple observations such as odor, temperature, and most importantly, color. However, some gas substances cannot be easily observed for their color. In such cases, determining the molar mass of the substance can prove to be helpful in substance identification.
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According to Purdue University’s department of chemistry, for many chemists, it is impractical to collect and measure gas because gases have small densities. However, because butane is not soluble in water, it allows for water to be displaced from a container. In doing so, it facilitates the collection of gas. In Dalton’s Law of partial of pressures, the total pressure in a container is equal to the sum of the gas collected and water vapor.
In 1834, physicist Emil Clapeyron wrote an equation that assisted many chemists in understanding the behavior of everyday gases. Clapeyron equation is recognized as the ideal gas law and is written as PV=nRT. It states that the product of a gas’s volume (L) and pressure (atm) is proportional to the product of the gas constant, moles (mol), temperature (K). The gas constant, “R” has an exact value of 0.0821 L*atm/mol*k. This equation is important because the number of moles, “n”, can be used to determine the molar mass of butane by rearranging the equation to n=PV/RT. In this experiment, the moles and mass will be required to determine the molar mass of butane in a butane lighter.
Experimental
To begin the experiment, the mass of a butane lighter was measured before using the butane gas to deplete the water to the 80mL mark on a graduated cylinder. Secondly, the temperature of the of the water was measured after waiting five minutes for the temperature to remain constant. Afterwards, the mass of the butane lighter was measured a second time to determine the displacement.
In calculating the molar mass, the ideal gas law was used to first determine the number of moles. Finally, the mass of the butane displaced was divided by the moles to eventually produce the molar mass of the butane. The experiment was repeated for four trials.
Results
Table 1: Data of molar mass experiment
Units
Trial 1
Trial 2
Trial 3
Trial 4
Initial Mass of Lighter
g
14.053
14.827
16.234
13.903
Final Mass of Lighter
g
14.273
15.060
16.346
14.071
Mass of Butane
g
0.220
0.233
0.112
0.168
Volume of Gas collected
L
0.101
0.084
0.094
0.082
Air temperature
K
293.000
293.000
293.000
293.000
Water temperature
K
293.000
293.000
293.000
293.000
Vapor pressure of H₂O
atm
0.023
0.023
0.023
0.023
Barometric Pressure
atm
0.988
0.988
0.988
0.988
Accepted Molar Mass of Butane
58.12g/mol
Certain data included in the table were given as a standard. That standard was used to compare the experimental result to. The accepted molar mass of butane, vapor pressure of H₂O and barometric pressure was given as a standard.
Table 2: Number of moles by the equation n=PV/RT for trials 1-4
Trial 1 Data
n=PV/RT (mol)
Trial 2 Data
n=PV/RT (mol)
P= 0.9649
0.004
P= 0.9649
0.003
V= 0.101
V= 0.084
R= 293
R= 293
T= 0.0821
T= 0.0821
Trial 3 Data
n=PV/RT (mol)
Trial 4 Data
n=PV/RT (mol)
P= 0.9649
0.004
P= 0.9649
0.003
V= 0.094
V= 0.082
R= 293
R= 293
T= 0.0821
T= 0.0821
The number of moles was calculated by multiplying the pressure and volume and dividing that product by the constant gas and temperature. The pressure was determined by subtracting the pressure of H₂O from the total pressure.
Table 3: Molar mass of butane calculations
The molar mass was calculated by dividing the mass of butane by the experimental number of moles. The average of all four trials resulted in a molar mass of 54.17g/mol.
Discussion
The experiment was designed to be able to facilitate substance identification by determining the molar mass of butane. The molar mass of butane was found by first calculating the number of moles using the ideal gas law, n=PV/RT. Next, the mass of the butane displaced was divided by the moles to finally give the molar mass of the butane. The average experimental molar mass resulted in 54.17g/mol while the accepted value was 58.12g/mol. This is a percent error of 6.779%. In the future, this experiment can be improved upon by not letting the butane gas escape the graduated cylinder. In doing so, it will allow for more accurate results when calculating.
Bibliography
“Identification of a Pure Substance.” Austin Peay State University Department of Chemistry, Chem 1111 Lab Handouts. Revision S18.
Finding the Molar Mass of Butane. University of Illinois Urbana-Champaign, www.chem.uiuc.edu/chem103/molar_mass/introduction.htm.
Collection of Gas Over Water. Purdue University, chemed.chem.purdue.edu/genchem/lab/techniques/gascollect.html.
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