edumeister_logo

Lesson 3 Lab: Pipetting and Self-Inflating Hydrogeshowing page 1-5 out of 5

Lesson 3 Lab: Pipetting and Self-Inflating Hydrogen Balloons
PART 1
Begin by viewing the following Thinkwell video
15.1.5 
CIA Demonstration: Pipetting
After you watch the above video, answer the questions below in sufficient detail:
(a) (1 pts) What is a pipette and for what is it used? Be specific.
Answer: A pipette is a lab tool used to transfer a specific amount of volume from a liquid
from one container to be moved to another (Yee, n.d.).
The pipette itself is a long glass tube
with a bulge in the center that captures most of the liquid. By taking liquid into the pipette,
you are then able to transfer the solution to the receiving flask.
(b) (1 pts) What is the difference between a graduated pipette and a volumetric pipette, and what
is the specific function of each?
Answer: A graduated pipette has different marking on the tube that is used to transfer
varying amounts of liquid, these are less precise than volumetric pipettes and also does not
have the bulge in the center (Yee, n.d.).
A volumetric pipette is used when wanting to
transfer a specific amount of liquid such as exactly 20mm(Yee, n.d.).
(c) (2 pts) What is the proper technique for using each of these instruments? What is the
importance of the "meniscus" in this technique?
Answer: The volumetric pipette has a fill line at the top of the thin tub to measure the
liquid which should be even with the minisques. A graduated pipette, has lines for any
desired measurement, such as 3mm, and you would line the minisques with the desired
volume measurement (Yee, n.d.).
There is also a tip at the bottom of the pipette that will
draw in the liquid. When ready to draw in the wanted liquid, the student would gently
attatch the pipette ball at the top, while it is squeezed, and this draws it in as you let go. If it
has too little liquid in it you will removed the bulb, covering the top with your finger, and
repeat with the bulb. By drawing in more liquid than what is needed, you will be able to
remove the bulb and let the liquid drain slowly, using your finger to cover the top of the
pipette. By being eye level with your measurement, you will be able to determine if the
meniscus is where it needs to be. A small amount of liquid is left in the tip, but do not drain
the drop from the tip of the pipette (Yee, n.d.).
The graduated pipette is filled in exactly
the same way, but does not need to be exactly on zero.
(d) (1 pts) Give a specific example of how you could use a pipette (if one were available), in
your own daily life, and explain which type of pipette you should use in your situation and why.
Answer: I like to do science experiments with my son, so this could prove useful in those
situations. We could use it to transfer a specific amount of liquid to another to determine
what would change. He is only a toddler so we like to do colors, by using the pipette to
draw in yellow coloring and adding it to blue, he would learn the reaction. By drawing in
different amounts of coloring, he would learn that this determines the shade that the colors
change to.
PART 2
Please watch the following Thinkwell video:
3.2.4 
CIA Demonstration: Self-Inflating Hydrogen Balloons
After you watch the above video, answer the questions below in sufficient detail:
(a) (3 pts) First, the instructor violated several safety rules in this video. List and explain/discuss
at least (3) safety rules he violated.
Answer: The instructor did not have on proper lab saftey gear such as gloves, goggles, or
lab coat. He was also eating during the experiment which is a big no.
(b) (2 pts) Why did these balloons inflate? Write and discuss a balanced chemical equation for
this reaction and use it to help explain your answer.
Answer: Chemical equation reaction between magnesium and HCL is:
Mg (s) + 2HCL (aq) = MgCl2 (aq) + H2 (g)
The 1st and 2nd flask balloons did not inflate as much because the amount of magnesum
was lower, about one-eightieth mol and one-fortieth mol. The magnesium within these
flasks acted as a limiting reagent (Yee, n.d.).
For the 3rd flask, the magnesium amount was
one-twentieth which equaled the stoichiometric ratio compared to the HCL arleady in the
beaker which allowed for almost all of the contents to react (Yee, n.d.).
This allowed the
balloon to inflate more than the other two. In the 4th flask, the magnesium amount was
one-tenth mol. So where in flask 1 and 2, the magnesium was the limiting reagent, in the
4th flask the HCL is the limiting reagent. As a result, the reaction in 4th flask produced
more hydrogen than the other flasks.
(c) (2 pts) What remained in the last (4th) flask (the one with the green balloon) at the end of the
reaction? Why? What does this demonstrate with regard to limiting reactant?
Answer: The last (4th) flask contained left over unreacted magnesium because of the HCL
limiting reagent (Yee, n.d.).
(d) (2 pts) Why was the same amount of HCl kept in every flask? Why didn’t the instructor vary
those amounts as well? Be detailed and specific in your answer.
Answer: Each flask had one-tenth mole HCL because this amount reacts with one-
twentieth of mole magnesium metal (Yee, n.d.).
This reaction produces one-twentieth mole
of hydrogen gas which inflates the balloons. Using varying amounts would change or
hinder the reaction.
(e) (2 pts) Why did the 3rd flask balloon (the red one) inflate more than the 1st flask balloon (the
purple one)? Incorporate the concept of limiting reagent in your answer.
Answer: The 3rd flask inflated more than the 1st flask because it was able to react
completely. This is because there was an exact stoichiometric ratio between the ratio of
magnesium and the HCL (Yee, n.d.).
(f) (4 pts) Suppose you ran this same reaction (using the balloon setup as seen in the video) on
your own with two different flasks. In Flask A, you reacted 5.10 g Mg with 0.447 mol HCl. In
Flask B, you reacted 24.21 g Mg with 0.998 mol HCl. Which ballon will inflate the most?
Explain (in detail) why, showing all work.
Answer: Equation reaction between Mg and HCl, resulting in MgCl and H is as follows:
Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g)
Information for flask A:
Molecular mass of Mg - 24.30 g/mol
Molecular mass of H2 - 2.02 g/mol
Moles of Mg = 5.10
= .2099 mol Mg
24.3
Since there is less Mg in this formula, Mg is the limiting regeant (Tro, 2006).
We can determine the theoretical yeiold for the reaction of H2 in flask A by the following
equation:
2099 mol Mg X 1 mol H2
=
.10495 mol H2
2 mol Mg
Information for flask B:
Molecular mass Mg - 24.30 g/mol
Molecular mass of H2 - 2.02 g/mol
Moles of Mg - 24.21
= .9963 mol Mg
24.3
In flask B, the limiting regeant is Mg since there is less of this than the HCl (Tro, 2006).
We can determine the therotical yeild for the reaction in flask B by the following equation:
9963 mol Mg X 1 mol H2
= .49815 mol H2
2 mol Mg
Since flask A produced .10495 mol H2, and flask B was able to produce .49815 mol H2, we
can determine that the balloon attatched to flask B will inflate the most because more
reaction was produced.
References:
Tro, N. (2011
). Introductory Chemistry, 5
th
ed
.: Upper Saddle River, NJ: Pearson.
Yee, G. (n.d.). Online Videos | Online Courses - Math & Science online video courses. Retrieved
from
http://my.thinkwell.com/cf/play.cfm