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Hi. I am doing the experiment "How Salty Does The Sea Have To Be For An Egg To Float?' for my science fair project this year. I'm in the process of doing my background research and defining the important words and concepts. Unfortunately though I'm having trouble finding the answers to what a relative concentration and an absolute concentration are. Can anyone define what they are for me? Thanks.

Hi there,

I agree it is not entirely clear in the background section why these terms are important for this experiment. The absolute concentration would be something like weight of salt per volume of water, for example 1 gram per liter. I think the point the material is meant to convey is that in some cases the absolute concentration is less important than the relative concentrations of two solutions being compared, so if solution A is 1 gram per liter and solution B is 2 grams per liter, solution B is twice as concentrated in relative terms as solution A. Perhaps this is so simple that it didn't even occur to you that this is what it means. Let me know if I can be of any further help.

Cheers,
Colin

HELP< HELP< HELP!!!
I'm a 7th grader doing this project and I am stuck on calculating the relative and absolute salt concentrations of each cup. I made the stock solution by pouring 1 cup of salt into 5 cups of water. Next I added 3/4 cup of my stock salt solution to cup 1 and 3/4 cup plain tap water to cups 2-5. After that I measured out 3/4 cup stock solution and added it to cup 2, measured out 3/4 cup of the solution from cup to 2 and added it to cup 3, measured out 3/4 cup of the solution from cup 3 and added it to cup 4. How do I find the relative salt concentrations of each cup? I know the amount of salt decreases with each cup since they become more and more diluted. Once I figure this part out how do I calculate the absolute salt concentrations of the cups? Please help me. My project is due soon for the science fair.

Hi,

You added 1 part of salt to 5 parts of water so the relative concentration of the salt is 1/5 or 20% by volume. The addition of a saturating amount of salt to water actually causes its volume to decrease slightly, but the amount is so small that we can disregard it.

Let’s set up the cups in order so we can keep track of the change in percent salt. ¾ cup = 6 fluid ounces (6 oz). Note that 6 fluid oz is NOT the same as 6 oz in weight! For scientific purposes, solids are usually measured by weight (grams), not volume. Water is measured in milliliters (mL) not ounces. One mL weighs on gram.

1. 6 oz 20% salt
2. 6 oz 20% salt + 6 oz water = 10% salt
3. 6 oz 10% salt + 6 oz water = 5% salt
4. 6 oz 5% salt + 6 oz water = 2.5% salt
5. 6 oz water only = 0% salt

So, what you just did is called a serial dilution you made a series of salt solutions from 20% to 0% by volume.

Now how can you determine the actual concentration of salt? The concentration of a solution is the weight of solute, the substance being dissolved, per unit volume or per unit weight. Scientists use the metric scale for weights and volumes but since you have used ounces we’ll stick to that scale. You could convert the weight of salt and the volume of water to metric units, but it isn’t necessary.

What you need to find out now is how much 1 cup of salt and 5 cups of water weigh. If you have a scale in your kitchen or at school you could measure out 1 cup of salt and weigh it. This would be the most accurate way to do it.
Or you can look it up online: http://www.aqua-calc.com/calculate/weig ... blank-fine
Depending on how fine or coarse it is, salt weighs about 10 oz per cup. So you see, 1 oz of salt by volume weighs more than 1 oz by weight. Now, how much does 5 c of water weigh? One cup of water weighs 8.35 oz (http://www.answers.com/Q/What_is_1_cup_ ... _in_pounds) so 5 c weighs 41.7 oz.

The formula for weight percent is the weight of the salt divided by the weight of the salt plus water times 100. For your stock solution that is 10 divided by 10 + 41.7 = 0.193 times 100 = 19.3%.

So, here are your cups again with the salt concentrations expressed as weight percent:

1. 19.3%
2. 9.67%
3. 4.83%
4. 2.42%
5. 0%

So, you see the percent by volume and the percent by weight are almost the same. How accurately you measure something depends on what you are doing. In this case the percent of salt does not need to be that accurate so the volume to volume calculation is adequate. http://dl.clackamas.edu/ch105-04/weight.htm

I hope this answers your question. If you have more questions please ask away.

Good luck!

Sybee

Hi Sybee (SciB),
Thanks for all your help you gave me on absolute and relative concentrations. I just have one more question about serial dilutions (I think anyway)...Cup 2 was where the egg first floated so now I need to make a serial dilution using this cup. I made more stock that has the concentration of cup 2. I want to make a dilution with smaller steps, reducing it by 25% with each cup. Do I do this by adding 3/4 cup of my stock solution to 1/4 cup of tap water in cup 2?

Hi,

Yep--you got it. The egg did not float in 5% but did float in 10% salt, right? So now you want to know the flotation % more exactly so you planned to mix 3/4 cup [6 oz] of 10% salt solution with 1/4 c [2 oz] of water. 6 divided by 8 times 10% = 7.5%

I would suggest, however, that you make at least TWO salt concentrations between 5% and 10%. If you take 6.4 oz of your 10% solution and mix it with 1.6 oz of water that will give you 8% salt. Now take 4.8 oz of 10% plus 3.2 oz of water and you will have 6%, if my calculations are correct. You could make even finer dilutions if you want to find out the exact percent to float an egg.

There is something called neutral buoyancy. When i go diving i don't want to float or sink, so i add weight to the weight belt until it just balances my buoyancy. Now I neither sink nor float but am weightless like on the space station! If you could find the exact percentage of salt you could make the egg have neutral buoyancy. Get a tall jar with a neck large enough to get the egg in and fill it with this salt solution and you can have the egg 'hover' wherever you want in the solution.

Have fun, take lots of pix and let us know how it turns out.

Sybee

I NEED HELP AGAIN
Hi Sybee (SciB)
I thought I was done with the questions but unfortunately Im stuck again. How do you calculate the densities of the egg in my experiment? I know that density is calculated by mass divided by volume but I dont know what numbers to use. the only calculations I have are the relative and sbsolute concentrations.
Also I wanted to make sure I did my 25% serial dilution and relative concentrations right?
Cup 1: 6oz. 10% salt
Cup2: 6oz. 10% salt + 2oz water.= 7.5%
Cup 3: 2oz. 7.5% salt + 2oz water= 5.63%
Cup 4: 2oz. 5.63% salt + 2oz water= 4.22%
Cup 5: 6 oz water only= 0% salt
Did I do that right?
For my relative concentrations I have...
Cup 1: 146 g NaCl/ 1.18 L x 1 mole/ 58.4 g =2.12 gpl (since the original dilution was 50% 146 grams is half the amount of salt in cup 1 from the original dilution. I started my 25% dilution from cup 2 of the original dilution because that is where the egg floated first. Should the amount of water also have changed to half or does it stay the same change from the original dilution? 1.18 L=5 cups
Cup 2: 2.12 x .25=0.53gpl
Cup 3: 0.53 x .25=0.13gpl
Cup 4: 0.13 x .25= 0.033gpl
Cup 5: 0 gpl
Did I calculate this correctly? I hope I didnt confuse you too much. Thanks again for all your help. I really appreciate it. I can see the finish line.

Hello. We noticed you had multiple threads going for the same topic. We merged them into a single thread. Please keep all questions to one thread so that our experts can best help you based on what has already been discussed.

Good luck, and thanks for using Science Buddies!

Hi,

OK. We got some problems here.

Your procedure for making the 25% serial dilutions starts out right for cup 2—6 oz is ¾ of 8. If you had continued with this formula you would have been correct. But instead you mixed EQUAL volumes of salt solution and water which cuts the concentration in HALF, not by a quarter which is what you want. The % that you listed, 5.63% and 4.22% are correct but you can’t get them by mixing equal volumes of solution and water. You have to use the ¾ to ¼ ratio [6oz to 2 oz] that you used for the 7.5% solution. Make sense?

In your procedure for relative concentration you say you put 146 g of NaCl [1/2 c] in 5 cups [1.18 L] of water. In your first post you said the original solution was 1 c salt in 5 c water which would make a 20% solution in cup 1, 10% in cup 2, 5% in cup 3, 2.5% in cup 4 and 0% in cup 5. Can you explain this better?

I am a little confused by the term ‘relative concentration’. When I did the weight to weight calculation that’s what I thought you were asking. Volume to volume first then weight to weight. Now I see you are expressing concentration of the salt solution in moles per liter [moles/L]. You are correct that 1 mole of NaCl weighs 58.4 g but the result is not in gpl but in moles per liter. A solution that contains 1 mole per liter is called a 1 molar solution or 1M for short.

Here are my calculations for all the different salt concentrations:
1. 20% salt, 4.24M
2. 10%, 2.12M
3. 7.5%, 1.59M
4. 5.63%, 1.19M
5. 5%, 1.06M
5. 4.22%, 0.89M
6. 2.5%, 0.53M

Now for your first question—how to calculate the density of an egg. It’s easy. First weigh the egg in grams then fill an 8 oz measuring cup that has milliliter marks to exactly 150 mL with WATER—not salt solution! The egg will sink and you can read the volume displaced. I tried it with a large egg and its volume was almost exactly 100 mL.

I hope this clears up your problems so you can finally cross the finish line and be done with your project.

Good luck!

Sybee

Hi SciB,
It's me again. In order to figure out the volume of the eggs for the densities I subtracted the volume of the (egg+water) from the 150 ml of water. When I placed each egg in the 150 ml of water it rose to 200ml so the volume of each egg would be 50 ml correct?

Now for my 25 % serial dilution my cups should go as follows:
Cup 1: 6oz 10% salt
Cup 2: 6oz 10% salt + 2oz. water= 7.5%
Cup 3: 6oz. 7.5% salt+ 2oz water= 5.63%
Cup 4: 6oz. 5.63% salt+ 2oz water= 4.22%
Cup 5: 6oz. of water only = 0%
Is this correct? Since you told me that the percent by volume and percent by weight are almost similar can't I use these %s as my relative salt concentrations?

Now for the absolute salt concentrations:
I need it to be expressed in moles per liter or ml. So what I did for my first set of cups was I used these:
I cup of salt = 292 grams
I cup of water= 237 ml.
I cup = 0.236588 liters
Remember that my original stock was made from 1 cup of salt dissolved in 5 cups of water. So...
Cup 1: 292 grams NaCl/ 1.18 liters x 1 mole/ 58.4 grams = 4.24mpl
Since the series was a 50% dilution I divided by 2 to get cup 2, and so on.
Cup 2: 4.24mpl / 2 = 2.12mpl
Cup 3: 2.12mpl/ 2 = 1.06mpl
Cup 4: 1.06mpl/ 2 = 0.53mpl
Cup 5: 0 mpl

So now that I'm doing a 25% dilution series can I use this same formula, but reduce the amount of salt by half since the new series starts with cup 2 from the original dilution (which as we know has half the amount of salt of cup 1 from the original dilution)? Cup 1 in the 25% dilution will have 146 grams NACl? The amount of water in liters will stay the same? I will reduce each cup by 25%?
I hope this explanation was more clear. I look forward to hearing from you soon. Thanks again.

Hi again,

Great! You’re back on track now and heading for the finish line!

Your egg density measurement is correct. The volume of water displaced by the egg is exactly equal to the volume of the egg. Now just take the egg’s weight in grams and divide it by the mL of volume and you will have its density. Sometimes you will see the density expressed as grams per cubic centimeter [g/cc]. I just wanted to tell you in case you didn’t know—one mL is almost exactly equal to one cc. I said ‘almost’ because the volume of water varies slightly with temperature. Only at 4 degrees C is 1 mL of water exactly 1 cc.

The 25% serial dilutions are correct and yes, you don’t need to do the calculations using weight because they are nearly identical to the volume %.

Your molarity calculations are right on. There’s just one thing I have a problem with and that is your use of the abbreviation ‘mpl’. I know what you mean by ‘mpl’ but I have never seen it used scientifically. If you are sure that mpl is a scientifically acceptable abbreviation then go ahead and use it, but I would use the standard moles/L or better just say 4.24 M, 2.21 M, etc. Scientists express concentration either in weight per unit volume like mg/ml or 10% or in molarity such as a 0.5 M salt solution or a 5 mM [millimolar] salt solution. One part of mastering the techniques of science is to learn the correct terminology. Terms and units in science have very precise meanings and if you use them incorrectly or use one that people are not familiar with then you are likely to be misunderstood.

So you are home free with your project. Congratulations! On to the next…

Sybee

Sybee I'm so excited!!! I can finally move on to doing my board for the science fair. Thank you so much for all your help. Your the best!!!

I am happy to help a young scientist. You might be the one to invent that anti-gravity device I keep hoping for!

So keep up the good work and we will continue as your online mentors.

Good luck!

Sybee

Hi. I'm an 8th grader doing the 'How Salty Does the Sea Need to Be for an Egg to Float' project. I'm stuck on the keywords section. I tried to look up the definition for relative salt concentration and absolute salt concentration on Google, but nothing popped up. Please jelp!! I can't start this experiment unless I define the words.