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BigDon
2010-Mar-16, 08:03 PM
If you had 6 tablespoons of salt in five cups of water, pulled out a cup and a half of the solution and replaced it with fresh water, then decided that was the wrong direction, how much salt would I need to add to get back to the original strength solution??

Fazor
2010-Mar-16, 08:06 PM
Assuming the salt was distributed evenly in the water, wouldn't it be 1.8 tablespoons?

Taeolas
2010-Mar-16, 08:06 PM
Not positive, since I'm more comfortable working in metric, but I believe you would need to add 0.8 tblspoons to bring it back.

6 tblspoons in 5 cups is 1.2 tblspoons per cup. When you take out 1.5 cups, you are left with 3.5 cups of water left and 4.2 tablespoons of salt left. So to get back you would need to replace the 1.8 tablespoons that is missing.

*edit* corrected my bad math.

uncommonsense
2010-Mar-16, 08:09 PM
If solution was not past saturation, then each of the 5 cups held 1.2 tbsps of salt. You removed 1.8 (1.2 + .6)tbsps, so replace the same amount.

NEOWatcher
2010-Mar-16, 08:11 PM
Not positive, since I'm more comfortable working in metric,
Just use measurement x and measurement Y. Since it's all ratios, no need for type of measurement.


but I believe you would need to add 0.8 tblspoons to bring it back.
Ooops. 6 - 4.2 = 1.8 :shhh:

Fazor
2010-Mar-16, 08:11 PM
When you take out 1.5 cups, you are left with 3.5 cups of water left and 4.2 tablespoons of salt left.
Except he said that he replaced the water taken out with fresh water. So you'd have 4.2 tbsp salt, but still 5 cups of water, leaving the salt deficient by 1.8.

I'd imagine in reality though, that the salt does not necessarily dissolve into the water with an even distribution, meaning you don't really know how much salt was taken out when you removed the salted water.

The Backroad Astronomer
2010-Mar-16, 08:11 PM
I think Taeolas has a typo, but I agree it should be 1.8 tablespoons.

Taeolas
2010-Mar-16, 08:12 PM
Yeah my bad. Teach me to do the 'quick' calculation in my head after calculating with the calculator. :P

BigDon
2010-Mar-16, 08:17 PM
Excellent speed guys.

Yeah, I'm tweeking the specific gravity of that lava lamp I'm trying to restore. I thought my particular issue was the water was too salty when it wasn't quite right. When I lowered the s.g it went right to fail mode.

So I'm going to to bring it back up to concentration and add an extra tablespoon to see where that gets me.

Thanks again.

Sam5
2010-Mar-16, 08:18 PM
If you had 6 tablespoons of salt in five cups of water, pulled out a cup and a half of the solution and replaced it with fresh water, then decided that was the wrong direction, how much salt would I need to add to get back to the original strength solution??


I would suggest pouring out the original mix and start over with 6 new tablespoons of salt and five new cups of water. Salt and water are quite inexpensive.

:)

BigDon
2010-Mar-16, 08:22 PM
Oh, because of it's solubility I'm using epsom salt.

I think you can disolve more than 25% the weight of water used in magnesium sulfate septawhosyawhatsis. Something like 27 grams per 100 grams of water.

BigDon
2010-Mar-16, 08:24 PM
I would suggest pouring out the original mix and start over with 6 new tablespoons of salt and five new cups of water. Salt and water are quite inexpensive.

:)

Ah, but my supply of de-ionized water is limited and the store far away at the moment.

orionjim
2010-Mar-16, 08:31 PM
1 Cup = 16 tablespoons
5 cups of solution = 80 oz
Or 6 oz salt + 74 oz of water
Ratio of solution = .081 salt to .919 water
You removed 24 oz of solution so you need to add back 24 oz made up of .081 salt and .919 water.
(.081 x 24) = 1.944 tablespoons of salt and (.919 * 24) = 22.056 oz of water.
But 1.8 like everyone else said would probably be close enough.


Jim

cjl
2010-Mar-16, 08:37 PM
1 Cup = 16 tablespoons
5 cups of solution = 80 oz
Or 6 oz salt + 74 oz of water
Ratio of solution = .081 salt to .919 water
You removed 24 oz of solution so you need to add back 24 oz made up of .081 salt and .919 water.
(.081 x 24) = 1.944 tablespoons of salt and (.919 * 24) = 22.056 oz of water.
But 1.8 like everyone else said would probably be close enough.


Jim
Your mistake was in the statement where you state that the 5 cups of solution contains 6 ounces of salt and 74 ounces of water. When you dissolve small quantities of salt in water, the volume of the water doesn't increase, despite the addition of salt. Therefore, the solution actually contains 80 oz of water and 6 oz of salt. Besides, it's unnecessary to use as complex of a method as you did to solve it.

When you remove 1.5 cups, you are removing (1.5)/(5) of the total solution, and assuming the salt is evenly distributed, you will also be removing (1.5)/(5) of the salt content. 1.5/5*6 = 1.8 ounces of salt removed (your value is slightly too high).

BigDon
2010-Mar-16, 08:50 PM
I would suggest pouring out the original mix and start over with 6 new tablespoons of salt and five new cups of water. Salt and water are quite inexpensive.

:)

AND I ended up doing that anyway. :)

Seems I was right the first time and decreasing was the way to go. At the new higher concentration everything bobbed to the top even though the lamp is cold.

So I started over and I'm using half the original amount of salt.

Now it has to heat at least an hour and a half.