Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Edher on February 21, 2005, 06:33:34 PM
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Saludos,
I'm having trouble starting this problem. I'm a bit baffled about the presence of the 8H20 in the chemical formula. What exactly does that mean?
Calculate [H30] and pH in saturated Ba(OH)2 (aq), which contains
3.9 Ba(OH)2 * 8H20 per 100 mL of solution.
Edher
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8H2O refers to the water of crystallisation. water molecules are present inside crystals of salt, if these crystals are made from salt solutions. the molecular formula tells you for every mole of salt present, there are 8moles of water present.
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Saludos,
I'm having trouble starting this problem. I'm a bit baffled about the presence of the 8H20 in the chemical formula. What exactly does that mean?
Calculate [H30] and pH in saturated Ba(OH)2 (aq), which contains
3.9 Ba(OH)2 * 8H20 per 100 mL of solution.
Edher
Saludos,
I'm still having trouble solving this problem.
I calculated the Molar Mass of the crystal (including water) it turned out to be 315.443 moles.
Then I calculated the Molarity by dividing the Molar Mass by the volume of the liquid and it turned out to be 0.2276M.
The I found the negative Log and I got 0.90.
However the correct answer according to my book is pH = 12.40 and [H3O] = 4.0 x 10^-13
Thank you,
Edher
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You are messing up several things.
1st - molar mass is 315.4 gram per mol, not 315.4 mol.
2nd - you didn't tell how many moles is 3.9g, but according to your concentration calculations you have found it to be 0.02276 mole - which is wrong.
3rd - barium hydroxide dissociation ends up with OH- ions, not H+ - so you have to use Kw in your calculations.
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You are messing up several things.
1st - molar mass is 315.4 gram per mol, not 315.4 mol.
2nd - you didn't tell how many moles is 3.9g, but according to your concentration calculations you have found it to be 0.02276 mole - which is wrong.
3rd - barium hydroxide dissociation ends up with OH- ions, not H+ - so you have to use Kw in your calculations.
I know that the molar mass of the crystal is 315.4 grams, when I typed this in the computer I accidentally typed moles. Nevertheless, I treated it as molar mass when I attempted to solve the problem.
The number of moles I obtained after doing the stoicheometry is 0.0123635 moles. (I obtained this by multiplying the given mass "3.9 grams" times 1 mole over it's molar mass.
Then I divided this by the volume of the solution (.1 L) and I got 0.123635 M.
Where exactly did I go wrong here according to you?
Edher
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Then I divided this by the volume of the solution (.1 L) and I got 0.123635 M.
Where exactly did I go wrong here according to you?
Previously you stated that calculated molarity is 0.2276M, which was wrong. 0.1236M is OK.
Go ahead.