Chemistry Help

[Vile Atheist]

I have a chemistry lab due next Thursday, so not a huge rush. I just wanted to finish it as early as possible and I'm stuck on this one part of the lab. Hoping for chem gurus to show themselves, damn it !

Basically we compared the cost-effectiveness of two brands of bleach (brand name and no-name) using the iodometric method. We looked at factors like the mass of available chlorine, cost per gram of Cl2, and mass percent of sodium hypochlorite.

There was a series of titrations for each brand and here was the procedure:

We diluted 10.00mL of one brand of bleach to 200.0 mL. We then took a 10.00 mL sample of the diluted bleach and added it to an Erlenmeyer flask. 25.0mL of water was then added along with 1.0 grams of potassium iodide. 5.000 mL of glacial acetic acid was added to the solution.

Titration then began. The resulting dark-orange solution was titrated with sodium thiosulfate until it turned pale yellow. 2.0mL of starch indicator was added to the pale yellow solution so that it turned dark blue. Titration then continued dropwise until the sodium thiosulfate solution turned the dark blue solution colourless.

The question I'm stuck on is: Is potassium iodide or sodium hypochlorite the limiting reactant? How do you know?

The lab demonstrator already told us it is NaOCl. I just don't know why.

Relevant stoichiometric equations are as follows:

OCl- + H2O + 2e- -> Cl- + 2OH-

OCl- + 3I- + H2O -> I3- + Cl- + 2OH-

I3- + 2S2O3(2-) -> 3I- + S4O6(2-)

I3- + Starch -> [I3•starch]-

2e- + [I3•starch]- -> 3I- + starch

OCl- -> 2S2O3(2-)

Cl2 + 3I- -> 2Cl- + I3-

I3- + 2S2O3(2-) -> 3I- + S4O6(2-)

2S2O3(2-) -> OCl-

2S2O3(2-) -> Cl2

OCl- -> Cl2



Thanks in advance for any responses I may receive!

 

[zenzero]

Friend CM,

Am not a chemistry student, still just googled it
and found this:
http://webspace.ship.edu/tascho/125/06-Redox.pdf

Best Wishes.
Love & rgds

 

[Vile Atheist]

Thanks zenzero! That was almost exactly what we did (a few minor differences). It mentions that potassium iodide the limiting reagent but doesn't say why. Although it did give me a couple other useful points of information I was able to use. Thanks a lot

 

[Tiapan]

The reagent with the least molar "excess" will be the limiting factor.

Sodium hypochlorite has formula weight of 74.45
Your Unknown is probably around 5% sodium hypochlorite

In our final reaction mixture

5% by 1 in 20 dilution = 0.25%
This further diluted by another factor of 4 =0.0625%
so per liter we have

5% asumed ie (50g/litre)
divide by 20 = 0.0025 (2.5g/litre)
divide by 4 = 0.000625 (0.625g/litre)
divide by 74.45 = 0.008395 Mol of sodium hypochlorite per litre

Potassium Iodide has a formula weight of 166.02 we use 1 gram or 1/166.01 Mole of Potassium Iodide in a solution of only 40m-50ml (1/20th of a litre) this is a pretty strong solution of Iodide

1
divide by 166.02 = 0.006023371
Multiply 20 = 0.120467414 Mol Potassium Iodide per litre
Even with the 3:1 molar ratio this is enough to handle up to a 0.04 Molar Sodium Hypochlorite Solution

The Potassium Iodide is obviously far more in molar excess, so the limiting agent is the Sodium Hypochlorite.

Cheers

 

[Vile Atheist]

Wow lol. Thanks Tiapan. I was definitely overthinking that...

 

[zenzero]

Friend CM,

Welcome but;
Thanks goes Rex and RF for this!

Love & rgds

 

[Vile Atheist]

See, what I did was this.

I have 10mL of original solution. The density of the solution is 1.084g/mL. So I figured I have 10.84g of solution.

The solution is 6% sodium hypochlorite, so multiply 10.84 by 0.06 = 0.6504g of sodium hypochlorite. Divide that by the molar mass of sodium hypochlorite to get moles of sodium hypochlorite: 0.008736983 moles of sodium hypochlorite = 0.008736983 moles of hypochlorite ion.

I added ~ 1 gram of potassium iodide (approximately. We used the pan balance approximation. Actually zenzero's link shows it, but unimportant...). 1g(1 mole/166.0028g) = 0.0006023994 moles of KI = 0.0006023994 moles of iodide ion.

I was having trouble at this point, knowing what reactions to use because the moles of iodide ion are significantly less than moles of hypochlorite ion.

 

[enchanted_one1975]

Yea but for those of us that have no clue what everyone has been talking about, did the lab go well?

 

[Vile Atheist]

Yep . The only thing that screwed me up is that I forgot to take into account that the number of moles is 1/20th of what was in the diluted sample.

But thanks for asking .

 

[enchanted_one1975]

Yep . The only thing that screwed me up is that I forgot to take into account that the number of moles is 1/20th of what was in the diluted sample.

But thanks for asking .

Hey I figure the more of you that do well in school the better chance I have that one of you will support me in my old age because I was so nice while you were a struggling student.

 

[Vile Atheist]

Hey I figure the more of you that do well in school the better chance I have that one of you will support me in my old age because I was so nice while you were a struggling student.

It's a nice sentiment, but I don't know how I personally can support you in your old age lol. I live in a different country.

 

[Vile Atheist]

I feel as if I should report my findings.

This lab found that it is more cost-effective to buy no-name brand bleach using the iodometric method. Javex costs twice as much per gram of bleach. However, Javex contains slightly more available chlorine (a measure used to indicate the efficacy of bleach) and a slightly greater mass percent of sodium hypochlorite. However, it is much more expensive than the no-name brand and the slightly increased efficacy does not justify the cost.

I had another lab comparing effectiveness and cost-effectiveness of antacids (using the back-titration method). Specifically I chose to compare Rolaids and Mylanta.

Rolaids is more effective and cost-effective at neutralizing stomach acid. But it contains calcium carbonate which when reacted with stomach acid, produces carbon dioxide gas. Mylanta, is half as effective at neutralizing stomach acid than Rolaids, but contains simethicone to combat stomach gas production and also its active ingredients are all hydroxide compounds, meaning no carbon dioxide gas is produced. While Mylanta is about 14 times more expensive to neutralize the same amount of stomach acid as Rolaids, it contains anti-gas components.

In terms of neutralizing stomach acid, Rolaids is more effective. In terms of combating stomach gas, Mylanta is more effective. Overall, Rolaids is the most cost-effective.

Note: A friend of mine chose Rolaids and Tums and did the same, using back-titration. Our Rolaids numbers were comparable, so they seem to be pretty accurate. He found Tums to be slightly less effective than Rolaids and slightly more expensive.

Conclusion: Rolaids > Tums > Mylanta.