Friday, September 28, 2012

Lesson 3 Lab: Pipetting and Self-Inflating Hydrogen Balloons



Student Name:  Ismail Kakembo                   
Student ID: 4181841
Date: 3nd Sept 2012
Course and Section Number: Chemistry


Lesson 3 Lab: Pipetting and Self-Inflating Hydrogen Balloons


PART 1

Begin by viewing the following Thinkwell video


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?

Answer:A pipette is a slender tube or pipe in which small amounts of liquids are taken up by suction to allow a chemist to remove, transfer or measure a substance.  It is used in a laboratory to measure a specific volume or varying amounts of liquid.



(b) (1 pts) What is the difference between a graduated pipette and a volumetric pipette, and what is the specific function of each?

Answer: Professor Gordon explains that a volumetric pipette is a pipe or tube which is used to transfer a precise amount of liquid such as 20.00 ml from one container to another whereas a graduated pipette is pipe or tube which is used to transfer varying amount of liquid. Volumetric pipettes are commonly used  in laboratories to transfer accurate amount of solution from one container to another.  They are also used in preparing solutions for titration that are used to determine how much of a particular substance is contained in a known volume of a solution.

(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:  According to professor Gordon, it is explained that a tip of a volumetric pipette must be calibrated in order to leave some amount of liquid in the tip. Then put the pipette into the beaker, allow the pipette to rest at the bottom or the middle of the liquid in the beaker. Grab the pipette bulb and squeeze it.  Rest the pipette bulb on top of the pipette and then start sucking the liquid from the beaker by releasing the pipette bulb. If the pipette bulb is released completing without getting the desired amount of liquid, cover the pipette with your index finger or thumb and squeeze the pipette bulb again. The right amount is captured when liquid fills up to the fill line and then it can be transferred  to the receiving flask. The same procedures apply to use of the graduated pipettes except that instead of filling to a fill line and transferring the total volume, the pipette is filled to a zero line and drained to the mark of the desired volume. A meniscus is important in this technique because it gives the interface between a liquid and a gas in the tube.

(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: When flying eggs at my house I tend to use lots of cooking oil. I could use a volumetric pipette to accurately measure and transfer cooking oil from the container to the flying pan.


PART 2

Please watch the following Thinkwell video:


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: In this video, Professor Gordon for got to wear a gown. This is a laboratory safety violation.

Secondly, professor Gordon can be seen in the video eating a banana. Laboratory safety precautions prohibit eating food in a laboratory. 

Lastly, Professor Gordon was not near a source of water incase he got contaminated he would have to go somewhere to look for a water source.

(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: The equation of the reaction between magnesium and hydrochloric acid resulting in magnesium chloride and hydrogen is:

Mg (s) + 2HCL (aq)  MgCl2 (aq) + H2 (g)

The balloons on the 1st and the 2nd flasks, inflated less because there was less magnesium 1/80 mol and 1/40 mol respectively. The magnesium in these two flasks acted as a limiting reagent. The amount of magnesium (1/20 mol ) in the 3rd flack was an exact stoichiometric ratio when compared to the Hydrochloric acid in the beaker hence making the balloon inflate a little more than flasks A and B because almost all the magnesium reacted. However, the 4th flask had more magnesium 1/10 mol and therefore HCL was a limiting reagent. The reaction in 4th flask (green balloon) will generate more hydrogen than the reactions in 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 unreacted magnesium because the HCL is the limiting reagent.


(d) (2 pts) Why was the same amount of HCl kept in every flask? Why didn’t the instructor vary those amounts as well?

Answer: Each flask contains one-tenth mole hydrochloric acid because this amount of hydrochloric acid reacts with one-twentieth mole magnesium metal to produce one-twentieth mole hydrogen gas.

(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 balloon inflate more because there was an exact stoichiometric ratio of magnesium (1/20 mol ) and that of Hydrochloric acid hence making the two to react completely. For this reason, the balloon on 3rd flask  inflates a little more than the balloons 1stand 2nd flasks.

(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 1.65 g Mg with 0.136 mol HCl. In Flask B, you reacted 2.21 g Mg with 0.098 mol HCl. Which balloon will inflate the most? Explain (in detail) why.

Answer: The amount of hydrogen in each flack

Given Mg – 1.65g
Molecular mass Mg = 24.30 g/mol
Molecular mass H2 = 2.02 g/mol
1.65 g Mg * (1 mol Mg/24.30g)*(1 mol H2/1 mol Mg)*(2.02 g H2/1 mol H2) = 0.14 g H2
Given HCL – 0.136 mol HCL
0.136 mol HCL*(1 mol H2/2 moles HCL)*(2.02 g H2/1 mol H2) = 0.14 g H2
1.65 g Mg reacting with 0.136 mol HCl will produce 0.14 g H2
In flask A there was no limiting reactant.

Flask B:
Given Mg – 2.21 g
Molecular mass Mg = 24.30 g/mol
Molecular mass H2 = 2.02 g/mol
2.21 g Mg * (1 mol Mg/24.30g)*(1 mol H2/1 mol Mg)*(2.02 g H2/1 mol H2) = 0.18 g H2
Given HCL – 0.098 mol HCL
0.098 mol HCL*(1 mol H2/2 moles HCL)*(2.02 g H2/1 mol H2) = 0.10 g H2
In this B, the hydrochloric acid is the limiting reactant, therefore
2.21 g Mg reacting with 0.098 mol HCl will produce 0.10 g H2.

The reaction in flask A will generate more hydrogen than the reaction in flask B, and therefore the balloon attached to flask A will inflate more



Reference

Gordon Yee. (2012). Introducing The Pipette. Retrieved From http://my.thinkwell.com/cf/playFull.cfm