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Mole-ratios-with-Mg-HCl by shitingting

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									Chemistry 10 Lab – Mole Ratios in a Reaction Producing H2 Gas

Aim/Hypothesis: The purpose of this investigation is to determine the mole ratio of H2 gas product
to Mg reactant in a chemical reaction.
If the mole ratio of H2 product to Mg reactant is ___________ as shown by the
balanced chemical equation, then a graph of moles of H2 vs. moles of Mg will appear
to be
___________________________________________________________________

Background:
Mg reacts with HCl in the following balanced chemical equation:

  A balanced chemical equation indicates                                                      Figure 1

   Gases from reactions can be collected over water in a graduated gas collection tube
(Figure 1).
   The volume of a gas is dependent on the pressure. When a gas is collected in a tube
immersed in a water reservoir, the pressure is dependent on the difference between the
level of water in the tube and the water reservoir, as in a manometer. A gas in a
manometer will be at atmospheric pressure when the liquid in the two arms of a
manometer are at equal levels. This can be done with a gas collection tube by
immersing the tube in a tall cylinder until the water level in the gas collection tube and
tall cylinder are equal (Figure 2). The pressure of the gas in the tube will then be
atmospheric pressure and the corresponding volume can be read.
   The combined gas law can be used to transform volume of gas under one set of
conditions to volume under a second set of conditions. It is based on


Apparatus/Materials:
                                                                                          Figure 2
Gas collection tube, water, 150 mL beaker, 250 mL beaker, 6 M HCl, magnesium
ribbon, tall 500 ml or 1000 ml graduated cylinder, thermometer, barometer, tray or dish for water
reservoir, sandpaper, balance (0.001 g or better)

Hazards: 6 M HCl is corrosive. Wear goggles, gloves, and apron. Wash hands after you are done.

Method:
   Practice filling a gas collection tube with water, immersing it in a tall cylinder, and checking how
the volume changes depending on the depth of the immersion. Practice until you do not lose any
water in the tube during the immersion process.
   Sand a length of magnesium ribbon until both surfaces are shiny, removing the oxide coating.
Check the volume of your gas collection tube. For 100 mL tubes, cut two different lengths of
magnesium ribbon between 0.5 and 3.0 cm and mass them to the nearest 0.001 g. For 50 mL tubes,
cut lengths between 0.5 and 1.5 cm. Fold the ribbons into a shape that will fit through the opening of
the gas collection tube.

   Fill your water reservoir. Into your gas collection tube, carefully pour in 6 M HCl so the gas
collection tube is about 1/5 full. Then carefully layer water over the denser HCl until the tube is
completely full and overflowing. Put your thumb over the opening, invert the tube, and immerse it in
your water reservoir. Quickly insert the magnesium up into the gas collection tube. It will float up
and react with the descending layer of HCl. Gently shake the tube to make sure the magnesium does
not stick to the sides of the gas collection tube. Once the reaction is complete, cover the opening of
the tube with your thumb so you can immerse the tube into your tall cylinder. Measure the volume
and temperature at the day's atmospheric pressure.
   Repeat with your other piece of magnesium. Clean your area and begin data analysis.
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Data Analysis

Part 1 – Your Personal Data
Calculate your moles of Mg.
Transform your H2 volumes to volumes at STP.
Transform your H2 volumes at STP to moles of H2 gas.
Tabulate your data and day’s conditions.

Part 2 – Combining Class Data
Get the spreadsheet of class data on the Chan Science Blog

Practice using formulas in the spreadsheet to quickly do the following calculations of the class data.

   Convert the masses of Mg into moles of Mg.

   Use the H2 gas volumes at STP to determine the moles of H2 gas produced.

Use the program Graphical Analysis 3 to graph Moles of H2 gas vs. moles of Mg to check your
hypothesis and support your answer to the aim.

A full report is due in 3 class periods.




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