**
**Purpose

To derive an experimental
value of R and compare it to the published value. A known mass of a metal is
reacted with hydrochloric acid to form hydrogen gas, H_{2}. The volume,
pressure, temperature and number of moles of the gas are calculated, allowing
for the calculation of R.

Introduction

The ideal gas law, PV=nRT
can be used to calculate the relative measure quantities of a sample of gas.
Knowing three of the four measured quantities (n,P,V and T) we can calculate the
fourth if R is known. In this lab we will experimentally determine R by
generating a sample of gas and measuring the four quantities and calculating the
gas constant by two methods. The procedure for this experiment is based on the
chemical reaction between Mg and HCl to produce H_{2(g)}:

1 Mg_{(s)}
+ 2 HCl_{(aq)} --->
1 MgCl_{2(aq)} +1 H_{2(g)}

The exact mass of Mg will be
measured and using the above equation, the **moles** of H_{2} produced can
be determined. (The reaction goes to completion) The experiment is done in a gas
collection tube which is calibrated, so the **volume** will be easily read. Before
measurements are taken, the gas system will be left to equilibrate to the
ambient room **temperature** which can thus be easily measured. Determining the
pressure however is a little more challenging. The hydrogen is collected in a
eudiometer tube over an aqueous solution and thus after the reaction is
complete, the pressure of the gas is that of the produced hydrogen and the vapor
pressure of water (which is easy to find in a table somewhere)

You
know the pressure of the atmosphere, you look up vapor pressure of
water, and you measure the difference in P between the inside of the
tube and the atmosphere, as measured by a ruler (P_{H2O(l)} ). Unfortunately, you measured P in mm of H_{2}O,
not mm Mg. To convert the pressure difference in terms of mm of Hg, you must
divide mm H_{2}O by 13.546 since Hg is 13.546 times as dense as water at
room temperature.

PV=nRT is
called the ideal gas law as it assumes some fundamental properties of gases,
namely that the gas molecules have no volume and that their attractive forces
are zero which is not always a valid assumption. You will be
calculating the value of R using PV=nRT. You will also need to
calculate the relative error obtained in your experiment:

**Prelab questions**: (To
be written on separately on a piece of recycled paper)

1. What is the vapor pressure of water at 25 deg C ? Where did you get this information?

2. Calculate the value of R
using the ideal gas law for the following
data: V_{dry H2} = 73.52 mL ; P_{dry H2} = 635 torr ; mass_{Mg}
(which gets you moles of H_{2}) = 0.0811 g ; T = 23.5 °C.

3. Your instructor will show
you how to do setup the experiment, but you must know what information you need
to determine for your three runs. Come up with some semblance of a data sheet.
You will *show* this to your instructor on the way in. No data sheet, no
lab.

~MEO 23Oct 2006