Acid base calculation?
August 8, 2013 7:33 PM Subscribe
How do you go about calculating the quantity of a base you would need to change the pH of a solution?
What I would know:
1) I have X litres of the acid
2) the pH of that acid
3) what I want the pH to be
4) the chemical formula of the base
How could I figure out how much of the base would I need to add to the acid to change the pH from one to the other?
For an example, if I had a 4.0 pH solution and wanted to make it 5.0 pH by adding Ca(OH)2 how would I figure out how much Ca(OH)2 would I need per litre?
What I would know:
1) I have X litres of the acid
2) the pH of that acid
3) what I want the pH to be
4) the chemical formula of the base
How could I figure out how much of the base would I need to add to the acid to change the pH from one to the other?
For an example, if I had a 4.0 pH solution and wanted to make it 5.0 pH by adding Ca(OH)2 how would I figure out how much Ca(OH)2 would I need per litre?
It's been years since I've done this stuff, but I don't think you have enough information. In particular, you would need to know what kind of acid you had; specifically, you would either need to know its initial molarity or its acid dissociation constant. (Knowing either one of these, along with the pH, tells you the other.)
posted by Johnny Assay at 8:09 PM on August 8, 2013 [1 favorite]
posted by Johnny Assay at 8:09 PM on August 8, 2013 [1 favorite]
Response by poster: Maybe more complex that I thought?
I've figured out from the wiki link the acid dissociation constant is related to the pKa?
If I use phosphoric acid in my example, the wiki says pKais
2.148, 7.198, 12.319
And then I'm confused why it's three numbers.
They've also got a chart that says a pH of 4.01 would be 10−4 mol/L and a pH of 5.00 is 10−5 mol/L. Is that something to go on? Like, that's a difference of 9x10-5 moles, and since Ca(OH)2 has 2 OHs for each loose H ion, I would need 9x10-5/2 moles of Ca(OH)2? Or is it not so simple as that?
posted by RobotHero at 8:54 PM on August 8, 2013
I've figured out from the wiki link the acid dissociation constant is related to the pKa?
If I use phosphoric acid in my example, the wiki says pKais
2.148, 7.198, 12.319
And then I'm confused why it's three numbers.
They've also got a chart that says a pH of 4.01 would be 10−4 mol/L and a pH of 5.00 is 10−5 mol/L. Is that something to go on? Like, that's a difference of 9x10-5 moles, and since Ca(OH)2 has 2 OHs for each loose H ion, I would need 9x10-5/2 moles of Ca(OH)2? Or is it not so simple as that?
posted by RobotHero at 8:54 PM on August 8, 2013
It's not so simple if it is pH 4 because of a weak acid (vinegar) as opposed to a strong acid. If its a weak acid, more of it would have been originally added to make the pH low, when you try to neutralize it, some of it will dissociate and resist the pH change (it will act as a buffer). Phosphoric has three pKa's because it can lose 3 H+'s, but each one is harder to remove than the previous. You only have to worry about the pKa's that are higher or less than 2 pH units (to be safe) lower. The common way to deal with this is the so called "ICE" table (the bane of chemistry undergrads) - Initial, Change, Equilibrium.
posted by 445supermag at 9:14 PM on August 8, 2013
posted by 445supermag at 9:14 PM on August 8, 2013
pKa is another way of expressing the acid dissociation constant; as in pH, the p stands for "negative log," so the pKa is the negative log of the Ka. We use this rather than Ka because it's more convenient: Ka values can go from 1010 for extremely strong acids to 10-50 for the weakest acids. Using pKa, we get a much smaller range of numbers, and makes it so that, as with pH, the lower you go down the scale, the more acidic the substance is.
Phosphoric acid has three pKas because H3PO4 has three hydrogens to lose; so, the first pKa is for when a hydrogen dissociates and the resultant ion is H2PO4-, the second for the next dissociation to HPO42-, and the third for the final dissociation to PO43-. The values go up as the substance becomes less acidic (less likely to give up a hydrogen).
posted by ocherdraco at 10:21 PM on August 8, 2013 [1 favorite]
Phosphoric acid has three pKas because H3PO4 has three hydrogens to lose; so, the first pKa is for when a hydrogen dissociates and the resultant ion is H2PO4-, the second for the next dissociation to HPO42-, and the third for the final dissociation to PO43-. The values go up as the substance becomes less acidic (less likely to give up a hydrogen).
posted by ocherdraco at 10:21 PM on August 8, 2013 [1 favorite]
This thread is closed to new comments.
posted by DevilsAdvocate at 8:08 PM on August 8, 2013