pH of salts?

[axp107]

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How do you find the pH of something that doesn't have hydrogens..

for example, how do you find the pH of CaCO3 in water?

 

[jheredia]

what are the choices?

i know it'll be basic because CO3 -2 will take up protons. i'm assuming the choices will have one basic, one neutral, and like 2 acidic options and u would pick the basic one

maybe i'm missing something though...

 

[axp107]

what are the choices?

i know it'll be basic because CO3 -2 will take up protons. i'm assuming the choices will have one basic, one neutral, and like 2 acidic options and u would pick the basic one

maybe i'm missing something though...

yeah the answer is 8, the only basic option. So, how'd you know it'd be basic..

do you first write the equation for its disassociation in water?

CaCO3 --> Ca2+ + CO3(2-)

Then what?

In general, the presence of a salt confuses me. So HCl is a strong acid.. taking away the H and adding an Na to become NaCl.. what happens to the acidity. What do the salts do

 

[scotties123]

what are the choices?

i know it'll be basic because CO3 -2 will take up protons. i'm assuming the choices will have one basic, one neutral, and like 2 acidic options and u would pick the basic one

maybe i'm missing something though...

Quoting EK: Na and Cl are conjugates of NaOH and HCl. As a salt, NaCl produces a neutral solution. NH4NO3 is made from conjugates of NH3 and strong acid HNO3. So, NH4+ is acidic and NO3- is neutral. As a salt, NH4NO3 is weakly acidic

I think of it like this: NH3 is weak base, HNO3 is strong acid. Strong acid + weak base = weakly acidic solution.
Correct me if im wrong, but this way has been working for me so far.

 

[slyloxy]

CaCO3----Ca2+ + CO32-
CO32- + H20-----HCO3- + OH-
This is how you get a basic solution


Another example NaCN
NaCN(aq)-----Na+ + CN-
CN- + H2O----- HCN + OH-
this concludes that HCN is a basic salt


Another example NH4Cl
NH4Cl----NH4+ + Cl-
NH4+ + H20--------NH3 + H3O+ therefore acidic salt


so in general salts coming from a strong acid will be acidic
and salts coming from a strong base will basic works always

 

[quiksilver87]

If it is a conjugate base of a strong acid then it is not basic nor acidic...This is because a strong acid dissociates completely, therefore a reverse reaction of picking up a proton is not possible.
If it is a conjugate base of a weak acid, then it can be basic. Because the revierse reaction is possible and it can pick up a hydrogen...

Carbonate anion is the conjugate base of bicarbonate, a weak acid. Just be sure to know your strong acids : HNO3, H2SO4, HCl, HBr, HI, HClO4, HClO3...

 

[axp107]

Ok thanks a lot guys.

So just to confirm:

So whenever faced a problem with a salt.

1) Disassociate it in water
2) React whatever disassociates (except spectator ion (Na+)) with water. If it produces, OH-, the original salt is basic.. or H+, the original salt we started off is acidic.

 

[slyloxy]

Ok thanks a lot guys.

So just to confirm:

So whenever faced a problem with a salt.

1) Disassociate it in water
2) React whatever disassociates (except spectator ion (Na+)) with water. If it produces, OH-, the original salt is basic.. or H+, the original salt we started off is acidic.

exactly

 

[RPedigo]

You have to remember your strong acids and strong bases.

When you put a salt in water, dissociate it into its component ions, and then see how acid/base would react with it.

Let's look at NaCl

1. Dissociate it
Na+ and Cl-

2. React it
Na+ can combine with OH- to make NaOH... this is a strong base, so it will dissociate completely. That means it can't really "absorb" the OH- from solution, so Na+ will just chill in the solution and do nothing.

Cl- can combine with H+ to make HCl... this is a strong acid, so it too will dissociate completely.

Since the cation Na+ cannot absorb OH- and the anion Cl- cannot absorb H+, we would expect the pH of a NaCl solution do be about 7. This is what is actually observed.



Let's look at your CaCO3

1. Dissociate it
Ca2+ and CO3--

2. React it
Ca2+ can combine with two OH- to make Ca(OH)2. This is a strong base and will dissociate completely. This means it cannot "absorb" the OH- from the solution, so it will not modify the pH.

CO3-- can combine with two H+ to make H2CO3. This is a weak base, so not all of it will dissociate. That means in equilibrium we will have some H2CO3, some HCO3- and some CO3--. We've effectively "absorbed" some H+ out of solution, so there is less free H+ now. That means that the solution is more basic.

Since we could not remove OH- from solution (since it'd dissociate back into solution since Ca(OH)2 is a strong base), but we can remove H+ from solution by having CO3-- react with it... we're going to get a basic solution.



Let me know if this makes any sense.