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<div class="moz-cite-prefix">On 7/3/20 10:27 PM, wrote:<br>
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cite="mid:2271621d1271499ea0662bec7d49418a@BN3PR01MB1969.prod.exchangelabs.com">
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<div dir="ltr">Hi Dr. Pounds,</div>
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<div>I was doing the lab report and had a question. How do you
calculate the initial [H2O2] for trial one? Is the initial
concentration for [I-] in trial two the same as the [H2O2]
concentration in trial 1? </div>
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<div dir="ltr">Thanks!</div>
<br>
</div>
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</p>
<p>The concentration kinetics lab requires you to do numerous
calculation. For all of the prelab calculations dealing with
questions like "What is the concentration of ??? in flask ???" you
just have to use the standard dilution formula <img
alt="$M_1V_1=M_2V_2$" style="vertical-align: middle;"
src="cid:part1.78FC3A40.ACECFBF0@mercer.edu"> where <img
alt="$M_1$" style="vertical-align: middle;"
src="cid:part2.F13D0461.CD9C1DFB@mercer.edu"> is the initial
molarity of the substance and <img alt="$V_1$"
style="vertical-align: middle;"
src="cid:part3.D5AC981F.B4E67D53@mercer.edu"> is the initial
volume of that substance and <img alt="$V_2$"
style="vertical-align: middle;"
src="cid:part4.C67A6355.2BDF818C@mercer.edu"> is the volume of
everything mixed together (which, by the way, is always 150 ml in
the prelab because it was written before we made the change this
semester to use 2 ml of starch). The hard part is determining the
rate of disappearance of <img alt="$H_2O_2$"
style="vertical-align: middle;"
src="cid:part5.407B54F8.934EA87B@mercer.edu">. You need to
understand something -- ALL of the rates you measure are for the
disappearance of <img alt="$H_2O_2$" style="vertical-align:
middle;" src="cid:part5.407B54F8.934EA87B@mercer.edu"> -- so
what I am saying here will apply to ALL of the rates you will fill
out on the report form. Here is what makes this difficult. The
reaction you are studying is <br>
</p>
<img alt="$2H^+ + H_2O_2 + 2I^- -> I_2 + 2 H_2O$"
style="vertical-align: middle;"
src="cid:part7.6E9588AB.49801344@mercer.edu">
<p> and we are using the clock reaction mentioned in the lab report
to "time" the reaction.</p>
<p><img alt="$I_2 + S_2O_3^{2-} \longrightarrow S_4O_6^{2-} + 2I^-$"
style="vertical-align: middle;"
src="cid:part8.14B0FE13.27E04EF0@mercer.edu"></p>
when the <img alt="$S_2O_3^{2-}$" style="vertical-align: middle;"
src="cid:part9.BA2043FD.BFE9C81D@mercer.edu"> gets used up, the
clock reaction can no longer regenerate <img alt="$I^-$"
style="vertical-align: middle;"
src="cid:part10.992E2635.5DF6D019@mercer.edu"> and the solution
will turn blue due to the presence of starch. So, what you are
measuring is the amount of time for all of the <img
alt="$S_2O_3^{2-}$" style="vertical-align: middle;"
src="cid:part9.BA2043FD.BFE9C81D@mercer.edu"> to be consumed --
and then you have to relate that back, through stoichiometry, to the
amount of <img alt="$H_2O_2$" style="vertical-align: middle;"
src="cid:part5.407B54F8.934EA87B@mercer.edu"> that was consumed.
Here is the good news, its a 1:2 ratio between the <img
alt="$H_2O_2$" style="vertical-align: middle;"
src="cid:part5.407B54F8.934EA87B@mercer.edu"> and the <img
alt="$S_2O_3^{2-}$" style="vertical-align: middle;"
src="cid:part9.BA2043FD.BFE9C81D@mercer.edu">, and the amount of <img
alt="$S_2O_3^{2-}$" style="vertical-align: middle;"
src="cid:part9.BA2043FD.BFE9C81D@mercer.edu"> in every reaction
vessel is the same (you add it as <img alt="$Na_2S_2O_3$"
style="vertical-align: middle;"
src="cid:part16.8EBBB966.35B511A0@mercer.edu">, but it immediately
ionizes). <br>
<p><br>
</p>
<p>So, based on what we find on the chart in the procedure manual,
you had 5.0 ml or 0.02 M <img alt="$S_2O_3^{2-}$"
style="vertical-align: middle;"
src="cid:part9.BA2043FD.BFE9C81D@mercer.edu"> that was then
diluted to 150 ml. This means that the concentration of <img
alt="$S_2O_3^{2-}$" style="vertical-align: middle;"
src="cid:part9.BA2043FD.BFE9C81D@mercer.edu"> just after mixing
but before reaction has to be<br>
</p>
<p>(5.0) (0.02) / (150) = 0.0006623 M <br>
</p>
<p> From the 1:2 stoichiometry, the rate of disappaerance of <img
alt="$H_2O_2$" style="vertical-align: middle;"
src="cid:part5.407B54F8.934EA87B@mercer.edu"> is thus...</p>
<p><br>
</p>
( (0.0006623) / 2 ) / time (in seconds). <br>
<p><br>
</p>
<p>ALL of your RATES will be calculated this way -- just plug in the
appropriate seconds. As always, let me know if you have any
questions. </p>
<p> If you still are having problems with the pre-labs, email or
text me and I'll try to help. <br>
</p>
<p><br>
</p>
<p>OR -- you could just use the spreadsheet that I made for you on
the class web page in the LAB section....</p>
<p><br>
</p>
<p><br>
</p>
<pre class="moz-signature" cols="72">--
Andrew J. Pounds, Ph.D. (<a class="moz-txt-link-abbreviated" href="mailto:pounds_aj@mercer.edu">pounds_aj@mercer.edu</a>)
Professor of Chemistry and Computer Science
Director of the Computational Science Program
Mercer University, Macon, GA 31207 (478) 301-5627
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