[Perrin]

Hi, I've been trying to solve a certain question for about an hour now, and I just can't get it right. The question is:

Arrange the following mixed solutions A, B and C in order of decreasing value of pH:
A: 15 ml of 0.1 mol/l H₂SO₄ and 10 ml of 0.1 mol/l NaOH.
B: 15 ml of 0.1 mol/l HCl and 10 ml of 0.1 mol/l Na₂CO₃.
C: 15 ml of 0.1 mol/l HCl and 10 ml of 0.1 mol/l NaOH.

Now, no matter how many times I revise, the answer I get is B = C > A, while the correct answer according to the answers page is B > C > A.

In A I found that there are 0.003 moles of H₃0⁺ from the H₂SO₄ and 0.001 moles of 0.001 OH^- from the NaOH. So I concluded that there is in total 0.002 moles of H₃0⁺.

In B I found 0.0015 moles of H₃0⁺ from HCl and 0.001 moles of OH^- from the Na₂CO₃, so in conclusion there are 0.0005 moles of H₃0⁺.

In C I found 0.0015 moles of H₃0⁺ from HCl and 0.001 moles of 0.001 OH^- from the NaOH, so in conclusion there are 0.0005 moles of H₃0⁺.

Now, because the volume of all solutions are the same, the less the quantity of H₃0⁺, the larger the pH, so I found B = C > A.

If someone can please check my answer, and see if it's correct it'll be very much appreciated!
Thanks.

[enahs]

My original hint sucked.

Write the balanced equations.

[Perrin]

The reaction I wrote for it was:

Solution:

Na₂CO_3(s)  2Na⁺_(aq) + CO₃^2-_(aq)

And the molar ratio:

1 : 2 : 1

Which means 0.001 moles CO₃^2- (like Na₂CO₃). Then with water:

CO₃^2-_(aq) + H₂O_(l)  HCO₃^- + OH^-

There for the ratio for CO₃^2- : OH^- is one, and we have 0.001 moles OH^-. Or at least that's what I think

[Perrin]

Is the equation I wrote in my previous post unbalanced?

[vhpk]

The reaction I wrote for it was:

Solution:

Na₂CO_3(s)  2Na⁺_(aq) + CO₃^2-_(aq)

And the molar ratio:

1 : 2 : 1
Which means 0.001 moles CO₃^2- (like Na₂CO₃). Then with water:

CO₃^2-_(aq) + H₂O_(l)  HCO₃^- + OH^-

There for the ratio for CO₃^2- : OH^- is one, and we have 0.001 moles OH^-. Or at least that's what I think

You don't use Kb value for carbonate anion, the rate can't be as you claimed.
Firstly, you should write the reaction that takes place in each solution. Compare the amount of each to find the concentration of proton or hydroxyl. Then find the pH value, not only find the initial concentration of proton

[Perrin]

Sorry, I'm pretty new to this stuff, so I still can't figure my mistake 

[Perrin]

Sorry for my ignorance, but what is the Kb value for carbonate anion? Why is the equation wrong?

[vhpk]

Sorry, I'm pretty new to this stuff, so I still can't figure my mistake 

As I said above, you should find what remained after the reaction, i.e whether proton react completely with CO32- to give CO2 or only to give HCO3-, whether there is any H+ in solution after reaction, whether there is any CO32- in solution after reaction.
If there is only H+ -> easily calculate pH value.
If there is carbonate or bicarbonate, you use Kb value of 2 ion to calculate the concentration of OH- by writing the dissociation.
Kb value: you can search on the web