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Topic: aqueous solution density  (Read 3632 times)

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Offline ytrewq

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aqueous solution density
« on: July 14, 2011, 12:44:21 AM »
Which of the following solution has greater density?

1. 100ml of saturated aqueous solution of Lif. (Ksp(Lif)=1.84*10^-3).

2. 100ml of saturated aqueous solution of Hg2F2. (Ksp(Hg2F2)=3*10^-6).

(We can assume that adding the dissolved doesn't change the volume of the solution).

Explanation would be appreciated.
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Offline Dan

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Re: aqueous solution density
« Reply #1 on: July 14, 2011, 03:35:27 AM »
Show your attempt. Forum Rules.

Do you know what Ksp means? If not, look it up.
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Offline ytrewq

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Re: aqueous solution density
« Reply #2 on: July 14, 2011, 03:44:17 AM »
Sorry. My attempt:

I've tried to find the solubility of both salts in 100ml of water.

As far as I understand the concentration of Lif is the square root of it's ksp=sqrt(1.84*10^-3)=0.043.

Now, 0.043=n/100ml, and by multiplying both sides of the equation by 25.941 (the Mw of Lif) I get that the solubility of Lif in 100ml of water is 1.12g/100ml (which according to wikipedia is wrong...). 

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Offline Dan

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Re: aqueous solution density
« Reply #3 on: July 14, 2011, 04:12:32 AM »
Quote from: ytrewq on July 14, 2011, 03:44:17 AM
As far as I understand the concentration of Lif is the square root of it's ksp=sqrt(1.84*10^-3)=0.043.

Ok, don't forget the units - mol dm-3

Quote
Now, 0.043=n/100ml, and by multiplying both sides of the equation by 25.941 (the Mw of Lif) I get that the solubility of Lif in 100ml of water is 1.12g/100ml (which according to wikipedia is wrong...).  

No, 0.043 = n/1000 mL

which gives a solubility of 1.12 g/L or 0.112 g/mL.

The solubility data on the wikipedia page is not a reliable source of information. Note that on Wikipedia the solubility of LiF is quoted as 0.27 g/100 mL at 18oC on the main LiF page, and then 0.16 g/100 mL at 20 oC on the solubility table page. This is contradictory, one of them is wrong. Wikipedia is useful but not always true - be cautious with it.

So... repeat for Hg2F2
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Offline ytrewq

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Re: aqueous solution density
« Reply #4 on: July 14, 2011, 05:21:43 AM »
Great!

So let me try Hg2F2:

concentration = sqrt(3*10^-6)=1.73*10^-3.
By multiplying by the mw (439), we get that solubility of Hg2F2 is 0.765177 g/L.

So to sum up, the solubility of Lif is greater then the solubility of Hg2F2, and since density is a function of mass and volume, and both solutions have the same volume, the greater the solubility the greater the density.

So my final answer is 1. Am I right?
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Offline AWK

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Re: aqueous solution density
« Reply #5 on: July 14, 2011, 10:34:06 AM »
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