If your lab partner had failed to look up the vapor pressure of water and assumed that it was zero, would your resulting value for Avogadro’s number be too high or too low? Thoroughly justify your response.
Calculating
Background
As discussed in lecture, electrochemistry allows stoichiometric calculations to be related
to current and time, if a chemical equation that relates electrons to reactants and products is
known. Recall that
into simpler substances. This particular experiment will involve the electrolysis of a sulfuric acid
solution to generate hydrogen gas. By measuring the amount of hydrogen produced by the
current and length of time of the electrolysis, it is possible to calculate a value for the Faraday
constant and thus also Avogadro’s number. The amount of hydrogen is determined by utilizing
the
electrolysis.
The amount of charge that has passed through the solution is measured in coulombs
(C). It is calculated by measuring the current (in amperes, A) that has passed through the
solution over a known time. Once the charge is known, it is possible, by knowing the amount of
hydrogen produced and the stoichiometry of the reaction, to determine Faraday’s constant (the
charge in coulombs, C, on one mole of electrons). Furthermore, since the charge of one
electron is known (1.60 X 10-19 C) Avogadro’s number can likewise be calculated.
Procedure
Set up the apparatus as shown on the demonstration table.
Place 100 mL of deionized water and 50 mL of 1 M sulfuric acid in a 250 mL beaker. Fill
the gas buret completely with this solution and invert it as demonstrated. Making sure that the
DC source is unplugged, attach a copper wire to the negative terminal of the DC source and
place the other end into the inverted mouth of the gas buret. Be certain that all of the
uninsulated wire is inside the buret so that all of the hydrogen is collected. Another copper
wire is simply immersed in the solution in the beaker. Read where the top of the solution is in
the buret and record this value.
Call your instructor to plug in and turn on the DC source, as you begin timing. Record
the current (it should start at approximately 0.20 amperes) every thirty seconds so that an
average current can be calculated. Do not touch the apparatus while it is plugged in! Continue
the electrolysis until at least 10 mL of hydrogen have been collected. Note the time and unplug
the DC source. Record the actual volume of hydrogen.
Measure the height (in millimeters) of the solution column in the gas buret above the
solution in the beaker (i.e. the distance from the top of the solution in the beaker to the top of
the solution in the column). Also, measure the temperature of the solution and determine the
air pressure in the room.
1. If your lab partner had failed to look up the vapor pressure of water and assumed that it was zero, would your resulting value for Avogadro’s number be too high or too low? Thoroughly justify your response.
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