Molar Mass of Butane
Cautions
Butane is toxic and flammable No OPEN Flames should be used in this expenment.
Introduction
When a new substance is prepared in the laboratory, its identity must be determined. Knowing the molar mass of the substance imits the list of possible identities. To determine the molar mass, two pieces of information must be known; the mass of the sample and the number of moles of substance present in that particular mass. For any sample. determining da mass is straightforward place a quantity on or in a container the balance, and read the mass i displays. Determining the number of moles is not as straightforward. In the case of a gaseous sample, knowing its pressure volume specific temperature provides enough information to determine the number of moles present Uhing The mass and number of moles, the molar mass can be determined.
One method of determining the volume of a gas sample at a pressure is to collect water using a setup similar to one found in most general chemistry textbooks. If the volume (V), temperature (7) and total pressure (Pa) of the collected gas are measured, the gas law constant (R) and the ideal gas law can be used to determine the moles (n) of hydrogen produced by the sample: PVRT (Equation 11
Determining the pressure of collected gas is not as straightforward as simply measuring atmospheric pressure. Samples of liquid waller in container always have a small amount of waller vapor present just above the surface (vapor pressure), which must be accounted for. Accounting for this small quantity of water vapor requires using Datton’s law of partial pressures:
(Equation 2)
Since the gaseous sample is collected over water in an enclosed flask, can assume the gas is saturated with water vapor at barometric pressure. Thus, the total pressure observed under these conditions is equal to the sum of the vapor pressure of water at that temperature and the pressure of the gas from the experiment. The vapor pressure of water is constant at various temperatures and here again, san usually be found in most general chemistry textbooks.
Butane is a gas at normal room conditions, however in disposable lighters it is a liquid. This phase change is achieved by putting a sample of butane under high pressure within the lighter. When the valve of the lighter is depressed it provides an opening. depressurizing the container and allowing the liquid butane to escape in the form of a gas. Butane is a hydrocarbon, composed of only carbon and hydrogen atoms connected by covalent bonds. The composition of butane leads to its physical and chemical properties; butane molecules do not interact significantly other or with other molecules. Hence, butane is a gas at room temperature and does not dissolve significantly in water. These properties make bullane an ideal sample for this particular
experiment.
In this experiment you will collect a butane gas sample in a container by the water displacement method, allowing direct measurement of the volume of butane gas collected. The pressure of the butane gas can be determined using Equation 2 above.
Vapor Pressure of Water Temp (°C) VP (torr)
16
17
18
19
20
21
22
22
23
24
25
27
28
29
30
13.6 14.5
15.5
16.5
17.5 18.6
10 21.0
22.4
237 25.2
26.7
28.3
30.0
31.6
The Correct Answer and Explanation is :
Correct Answer:
To determine the molar mass of butane (C₄H₁₀), we use the ideal gas law:
[
PV = nRT
]
where:
- ( P ) = pressure of butane (in atm)
- ( V ) = volume of collected butane gas (in liters)
- ( n ) = moles of butane
- ( R ) = ideal gas constant (0.0821 L·atm/mol·K)
- ( T ) = temperature (in Kelvin)
Step-by-Step Explanation:
- Determine the pressure of butane gas:
Since the butane is collected over water, the total pressure includes the atmospheric pressure and water vapor pressure. Using Dalton’s Law of Partial Pressures: [
P_{\text{total}} = P_{\text{butane}} + P_{\text{water vapor}}
] Rearranging: [
P_{\text{butane}} = P_{\text{total}} – P_{\text{water vapor}}
]
- The total pressure is measured using a barometer.
- The vapor pressure of water at the experimental temperature is taken from the provided table.
- Measure the volume of butane gas:
The butane gas volume is collected using the water displacement method, where the amount of gas is measured in mL and converted to liters (L). - Convert temperature to Kelvin:
[
T(K) = T(°C) + 273.15
] - Calculate the moles of butane gas:
Rearranging the ideal gas law to solve for ( n ): [
n = \frac{P V}{R T}
]
- Use the corrected butane pressure in atm.
- Use the measured volume in L.
- Use the gas constant ( R = 0.0821 ) L·atm/mol·K.
- Use temperature in Kelvin.
- Calculate molar mass:
Finally, molar mass is determined by: [
M = \frac{\text{mass of butane}}{\text{moles of butane}}
] The mass of butane is obtained by weighing the lighter before and after releasing the gas.
Explanation (300 Words):
In this experiment, we determine the molar mass of butane using its mass and moles. The mass is measured by weighing a butane lighter before and after releasing gas into a container, ensuring that only butane escapes. The moles of butane are calculated using the ideal gas law, where we measure the volume, temperature, and pressure of the collected gas.
Because butane is collected over water, we need to correct for water vapor pressure using Dalton’s Law. The atmospheric pressure (measured using a barometer) includes the butane gas pressure and water vapor pressure. Using the provided vapor pressure table, we subtract the water vapor pressure at the given temperature from the total pressure to get the pure butane gas pressure.
The volume of butane gas is measured using the water displacement method, which captures the gas in a graduated container. The temperature of the gas is measured in Celsius and converted to Kelvin.
Using the ideal gas law, we solve for the number of moles of butane:
[
n = \frac{PV}{RT}
]
Finally, we determine the molar mass of butane by dividing the mass of butane released by the number of moles calculated.
This experiment effectively demonstrates how gas laws help determine the identity of a gas. Butane is ideal for this method because it is nonpolar, does not dissolve significantly in water, and follows the ideal gas behavior under laboratory conditions.