INTRODUCTION The ideal gas law has many applications. Because it relates several

Question

INTRODUCTION The ideal gas law has many applications. Because it relates several physical properties of a substance in the gas phase, the ideal gas law equation can be used to determine any one of these properties if all the others are known. In this experiment, the Dumas Method will be used to determine the mass of a gas then using easily measured values of pressure, volume and temperature, the molecular weight of an unknown compound will be determined. RELEVANCE As stated above, this experiment focuses on the use of gas laws to determine the molar mass of an unknown compound. While this is an important calculation for research or a similar purpose, it does not fully impart the value of these laws. Gas laws and the general concepts of gas volumes and pressures are vital for anyone deciding on a career in chemistry because many of our models of molecular behavior were developed as a result of early studies of gases. Gas laws are even more important to those focusing on medicine. For example, oxygen saturation in the body is controlled by both internal and external pressures. Why is it harder to breathe at higher altitudes? What is hypoxia? What happens to a patient who receives too much oxygen? Not enough? And how do you as the doctor or nurse control the amount delivered? None of these questions can be answered without a firm understanding of the gas laws. BACKGROUND The Ideal Gas Law states that PV- nRT, where P-the pressure of the gas in atmospheres, V- the volume of the gas in liters, n - the moles of gas, T-the temperature of the gas in Kelvins, and R- the gas constant. The gas constant R is the same for all gases, or mixtures of gases, L atm mol K and it has been experimentally determined to be 0.0821 A rearrangement of the Ideal Gas Law allows the calculation of the number of moles in a sample. PV n- RT 2013 by bluedoor, LLC 97

Explanation / Answer

1. Calculate average mass of unknown liquid collected in the flask in 3 trials:

Trail 1: mass of uknown liquid = final mass of liquid - initial mass of liquid = 88.42 g - 88.13 g = 0.29 g

Trail 2: mass of uknown liquid = final mass of liquid - initial mass of liquid = 91.79 g - 90.35 g = 1.44 g

Trail 1: mass of uknown liquid = final mass of liquid - initial mass of liquid = 87.87 g - 87.53 g = 0.34 g

Average mass of unknown liquid = (trial 1 + trial 2+ trial 3)/3 = (0.29+1.44+0.43)/3 = 2.16/3 = 0.72 g

2. Use the ideal gas law, to determine the mol wt of unknown gas in the flask. Assume the temp was 1000C

The ideal gas law is PV = nRT,

to identify the molar mass we can rearrange this equation as shown in the lab report as follows:

MW = (mass*R*T)/PV

where Pressure of the unknown gas = atmospheric pressure - pressure of water vapor at 1000C

Pressure of the unknown gas = 757 mm of Hg = 0.996 atm

average mass of unknown gas = 0.72 g

Gas Constant R = 0.08205 Latm/mol.K

Temperature = 100 + 273 = 373 K

volume of the flask = 150 ml = 0.150 L

MW = (mass*R*T)/PV =( 0.72g * 0.8205 Latm/mol.K * 373 K)/(0.996 atm * 0.150) = 147.5 g/mol

Identify the unknown from the list:

Based on the MW obtained the unknown should be Octane which has a molar mass of 114.2 g/mol

However the result can be very misleading in this case as the second trial has a huge variation in mass of unknown compared to the rest. 1.44 g in comapirison to 0.29 and 0.34 g.

Is the assumption of 100 deg C, a fair assumption:

This would be a fair assumption as long as there is not heat loss from the flask and the flas act as a perfect caloriemeter. In case the heat is lost then the temepature considered is higher than what it should be, leasding to a higher molar mass than the actual.

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