# Molar Mass report

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```					          Report Tutorial for Gas Laws: Molar Mass Determination of a Metal
(Palko 10-28-05)

In your report, use all the headings that appear below in BOLD. Bulleted and italicized notes

Purpose: To identify an unknown metal from its calculated the molar mass by measuring the
mass of a metal sample and then the volume, temperature and pressure of the gas produced
when the metal reacts completely with hydrochloric acid.

Procedure:
 Make a list of the students in your group and describe the major roles they played in
the experiment.

 Indicate the unknown number or letter of the metal you were assigned if this
information was provided. In some cases, only one metal is used.

Table of Data For Part II (reproduce this table in your lab notebook and turn in a copy of
your notebook entry with the report)
Trial Number
Mass of Metal Sample (g)
Temperature of Gas (°C)
Initial Pressure of Gas (torr)
Final Pressure of Gas (torr)
Net Pressure of Gas (torr)

Table of Results for Part II
 Create the following results table:

Trial Number
Moles of Gas = Moles of Metal (moles of M)
n = PV/RT (R = 0.08206 L*atm/K*mol)
Molar Mass of Metal
MM = g of M / moles of M

Calculations: (include)

   Example calculation of the moles of gas (and metal), the molar mass of the metal.
Conclusions

1. Calculate the average value for the molar mass for the class. This may be done for you
on the class data summary spreadsheet.
2. Use this average value as the accepted value and calculate a percent error for each of
3. How large is the range of molar mass values?. What was mesured during the lab? Does
it appear that the variation in the molar masses is due to measurement uncertainties?
If yes, what measurement seems to have the greatest uncertainty? If no, what other
source(s) of error could explain any discrepancies in your results?
4. Use your average value for the molar mass to identify the metal.
5. Compare the experimental molar mass of the metal with the actual molar mass of the
metal you’ve identified as the unkown. Be specific. Do not answer simply “They are
close” without explanation or evidence to support your statement. Does your selection
of metal make sense? For example, it is unlikely that sodium would be the unknown
metal due to discrepancies in the appearance of the unknown and sodium.
6. How would using a sample of metal that had a larger molar mass but mass equal to the
one you used affect the data collected? (Answer in terms of the pressure change that
results and the mole relationships in the reaction equation)
7. Create a list of metals that could NOT be identified in this manner since they will not react with
HCl. (Hint: Check the activity series).

An alternative method to determine the molar mass of the metal reacted would be to collect
the gas using water displacement. You can see a set up that uses this method on page 372 in
your book. Although the experiment described there involves the decomposition of
potassium chlorate, you could modify this to work for the reaction of a metal with an acid as
in our experiment.
(a) Draw a picture with labels that shows how you would modify this set up to work with
our reaction instead of the decomposition reaction shown.

(b) Use the sample data below to determine the molar mass and identity of the metal
reacted:

0.056 g of a metal is reacted with excess HCl to produce 22.8 ml of hydrogen at 25 C
and a room pressure of 725 torr. The vapor pressure of water at this temperature is 24
torr.

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