LexusHills
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Variables in My Experiment

Postby LexusHills » Wed Jul 15, 2015 6:17 am

I have a summer assignment which I would like to have done by July 22nd. I did the "How Salty Does the Sea Have to be For an Egg to Float?" experiment, but I have questions. I need a hypothesis in the "If...then..." format, which I understand completely, but I really can't tell exactly what my dependent variable is. The independent variable is the amount of salt, but based on the experiment, I can't tell how much salt was actually in the cup!

I learned what all of this was back in middle school, but I really don't know how I can finish this project without this information. Is there any way you could help me a bit?
Last edited by LexusHills on Wed Jul 15, 2015 2:11 pm, edited 1 time in total.
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HowardE
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Re: Variables in My Experiment

Postby HowardE » Wed Jul 15, 2015 6:36 am

Let's see if this helps.

What did you manipulate about the salt? The concentration of the salt solution? If you _____ the concentration of a salt solution, then an immersed egg will [float/sink] [more/less]. How did you determine that the egg was either floating or not? You considered it floating when some event happened or measurement threshold was met. You conclusion might then be that a concentration of ____ will cause a fresh egg to float in salt water.

viewtopic.php?f=25&t=5568 is a discussion from a while back that asked the same question so I'll refer you to that. Read through that and what I wrote above and see if that helps.

Howard

LexusHills
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Re: Variables in My Experiment

Postby LexusHills » Wed Jul 15, 2015 1:56 pm

I put five cups of water in a bowl and mixed it with one cup of salt, until the salt was completely dissolved. I then took 3/4 cup of that mixture and placed it in a smaller cup, which had enough slat to make the egg float.
To formulate my hypothesis, I need to know about how much salt was in the cup that the egg floated in and I can't seem to figure that out.
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HowardE
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Re: Variables in My Experiment

Postby HowardE » Wed Jul 15, 2015 2:22 pm

donnahardy's response (the third answer) in the thread viewtopic.php?f=25&t=5568 spells it very clearly. You need to know the volume of the water in liters (you have it in cups so you can do the conversion) and the mass of the salt in grams (you have it measured in cups so you can just weigh a cup of salt on a scale and convert that to grams). Then do the math as explained in that answer. That's one of the clearest examples I can find and it's right here on this forum.

LexusHills
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Re: Variables in My Experiment

Postby LexusHills » Thu Jul 16, 2015 8:12 am

I know how much salt I have now for my assignment, and I think I have a hypothesis now, but I will come back if I discover that I need something else for my assignment. Thank you for your time.
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LexusHills
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Re: Variables in My Experiment

Postby LexusHills » Thu Jul 16, 2015 10:03 am

If I have 3/4 cup of water, will an additional 1/4 cup of salt add enough volume to make the mixture a full cup? If it's not a full 1/4 cup being added, can you tell me how much will be added?

Tell me if you need me to clarify. I'm not really sure how to explain what I mean by this.
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Re: Variables in My Experiment

Postby HowardE » Thu Jul 16, 2015 1:44 pm

If I understand your question, you want to know if you start with a measured 3/4 cup of water and add 1/4 of salt will you end up with a full cup of salt solution?

You shouldn't. The salt (or at least some it) will dissolve in the water and the salt molecules fit themselves in the space between the water molecules. The combination of the two things will yield less volume than the two measured separately. To do the math for these experiments, you have to measure the salt and the water separately. For the dilution calculation you want to measure the mixed solutions.

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Re: Variables in My Experiment

Postby LexusHills » Fri Jul 17, 2015 7:15 am

Okay, that's what I thought. I was sure of the answer, but I just needed an explanation.
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Re: Variables in My Experiment

Postby LexusHills » Fri Jul 17, 2015 7:35 am

I've read those posts from your link a hundred times by now, but I need those metric units converted since moles are quite a bit higher and more advanced than I am, but I don't know how to convert them from mole to liter to cup. I have tried multiple conversion websites, but I am getting ridiculous answers that I know are not what I'm looking for. I've looked up a lot about moles too and they are advanced high school science that I don't have time to learn.
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tdaly
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Re: Variables in My Experiment

Postby tdaly » Fri Jul 17, 2015 8:36 am

Hi LexusHills,

HowardE has given you some good advice. The topic thread that Donna Hardy responded to is also quite thorough. I'm not sure that I understand what your questions are. The more details you can provide about what you are having difficulty understanding, the easier it will be for us to help you.

With that caveat, here are some responses to your questions.

RE: Variables (from your post on the other topic). You said in your very first post on this thread that the independent variable in this experiment is the amount of salt (specifically, the concentration of salt). That is correct. The dependent variable is the thing that changes in response to the independent variable. Usually the dependent variable is something you observe or measure. Re-read steps 5, 6, and 10 of the procedure. What are you observing in these steps? That is your dependent variable. The controlled variables are the things that you keep the same. As you re-read the procedure, look for things that are mentioned in the procedure but that do not change. For example, the procedure says to use eggs. Does the procedure say to change the type of egg you are using? No, it does not. So, the type of egg is one of the controlled variables. There are other controlled variables, too. Please post back with what you think the independent, dependent, and controlled variables are, and we will help you from there.

RE: Hypothesis (also from your post on the other topic). Usually you write out your hypothesis before doing the experiment. So, it isn't essential to know how much salt is in 1 cup to write your hypothesis. HowardE's first response included this fill-in-the-blank sentence: "If you _____ the concentration of a salt solution, then an immersed egg will [float/sink] [more/less]." You can either increase the concentration of salt or decrease the concentration of salt. Hence, another way to write HowardE's sentence would be, "If you [increase/decrease] the concentration of a salt solution, then an immersed egg will [float/sink] [more/less]." For each pair of words in brackets, choose one of the words based on what you think (or observed) will happen in the experiment. That sentence will then be a complete hypothesis. Please post back with your hypothesis and we will help you from there.

RE: Unit conversions. It would be helpful if you could post an example of what you put into the website and what value the website gave back to you. That will help us troubleshoot the conversions. Why do you want to convert your concentrations to moles? If you haven't had a chemistry class yet, I would think that leaving your concentrations in g/mL would be sufficient. Has your teacher asked you to convert the concentrations to molarity or molality?
All the best,
Terik

LexusHills
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Re: Variables in My Experiment

Postby LexusHills » Fri Jul 17, 2015 12:03 pm

I didn't actually want to convert everything to moles, but the link that HowardE gave me took me to a post where the expert was using moles (which I had personally never heard of until I saw those posts) and other metric units. I wanted to convert the moles to something I could work with, but I couldn't find any useful websites that I could get the answer that I was looking for from.

I have gotten my hypothesis and all my variables, and I have completed all my work for this experiment, but I have one more experiment and a lot of other work to do for school before the summer ends.
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LexusHills
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Re: Variables in My Experiment

Postby LexusHills » Fri Jul 17, 2015 12:08 pm

My teacher did not give us many specifics for this project. I was told to complete an experiment and organize it out with the variables, a conclusion, data tables, graphs, etc. This is a new teacher who I've never met or even seen before. I only learned about her when I received the assignment from her.

Donna Hardy's response was very deep, but also a bit confusing. I could make some sense of it, but not enough that it really helped me.
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Re: Variables in My Experiment

Postby HowardE » Fri Jul 17, 2015 12:53 pm

Donna Hardy's example of absolute concentration went like this: (quoting from Donna Hardy's response)
1 cup of salt weighs 272 grams
4 cups of water is .944 liters
Salt, or sodium chloride, or NaCl weighs 58.4 grams per mole

272 grams NaCl /.944 liters x 1 Mole/58.4 grams = 5 Moles per liter (your starting concentration)


To add to that, one standard measuring cup of salt has a mass of 273 grams (http://convert-to.com/456/table-salt-amounts-converter.html).

You started with 5 cups of water which is 1.25 * 0.944, or 1.18 liters.
One cup of salt is 273 grams or 273/58.4=4.68 moles. You concentration in moles/liter then is 4.68/1.18 or 3.96 moles per liter.
If you prefer grams/liter, then you have 273/1.18 or 231.36 grams/liter.

It's really just that simple. You then did the dilution from that stock solution. Can you fill in the table?

-------------------------------------------------
|Stock | 3.96 moles/liter|
| 1st dilution | |
| 2nd dilution | |
| 3rd dilution | |
etc
Last edited by HowardE on Fri Jul 17, 2015 1:18 pm, edited 1 time in total.

tdaly
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Re: Variables in My Experiment

Postby tdaly » Fri Jul 17, 2015 1:06 pm

Hi Lexus Hills,

Sounds like a rather stressful situation! I am glad that you were able to get your variables and hypothesis worked out. It sounds like you are all done with this experiment. But, if you are still working on it then you may find it easier to avoid working with moles. Instead, stick with g/mL. In those units the concentration is (# grams of salt)/(# of milliliters of water).

If you need more help with this project, feel free to post back on this topic thread. If you would like help with your new project, you'll probably want to start a new topic thread for the new experiment.
All the best,
Terik


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