Acid &Base: Calculate PH of HOCl & Net Ionic Eq.

[APCalculus]

Hypochlorous acid, HOCl, is a weak acid in water. The Ka expression for HOCl is shown below:
[H3O+][OCl-]/ [HOCl]= 3.2 X 10^-8

1) write a chemical equation showing how HOCl behaves as an acid in water.
i got: HOCl + H2O = H3O + OCl

2) calculate the PH of a 0.175 M solution of HOCl.
i got:
HOCl + H2O = H3O + OCl
Initial: 0.175 0 0 0
Change: -x -x +x +x
Equilibrium: 0.175-x -x +x +x

am i doing it right? how do u calculate the PH from x?


3) write the net ionic equation for the reaction between the weak acid HOCl and the strong base NaOH.
I got: HOCl + NaOH = H3O +Na +OH
is it right?


4) In an experiment, 20 mL of 0.175 M of HOCl is placed in a flask and titrated with 6.55 mL of 0.435 M NaOH.
A)Calculate the number of moles of NaOH added.
I did: (6.55/1000) x (0.435 M) = 0.0025

B) Calculate [H3O+] in the flask after the NaOH has been added.
where do i start? i don't get what the question is asking.

C) Calculate [OH-] in the flask after the NaOH has been added.
where do i start? i don't get what the question is asking.

thank you.

 

[APCalculus]

bump, anyone?

 

[Gokul43201]

Hypochlorous acid, HOCl, is a weak acid in water. The Ka expression for HOCl is shown below:
[H3O+][OCl-]/ [HOCl]= 3.2 X 10^-8

1) write a chemical equation showing how HOCl behaves as an acid in water.
i got: HOCl + H2O = H3O + OCl

You've forgotten to include the charges on the product ions.

2) calculate the PH of a 0.175 M solution of HOCl.
i got:
HOCl + H2O = H3O + OCl
Initial: 0.175 0 0 0
Change: -x -x +x +x
Equilibrium: 0.175-x -x +x +x

The concentration of water initially is not zero, but that's not important for the calculation - it is assumed there's a sufficient excess of water to not matter.

So you've got [HOCl] = 0.175-x, [H3O+] = x = [OCl-]

Now go back to the question and look at the expression for the Ka of HOCl.

3) write the net ionic equation for the reaction between the weak acid HOCl and the strong base NaOH.
I got: HOCl + NaOH = H3O +Na +OH
is it right?

Well, what are you supposed to get when you neutralize an acid with a base?

4) In an experiment, 20 mL of 0.175 M of HOCl is placed in a flask and titrated with 6.55 mL of 0.435 M NaOH.
A)Calculate the number of moles of NaOH added.
I did: (6.55/1000) x (0.435 M) = 0.0025

Correct.

B) Calculate [H3O+] in the flask after the NaOH has been added.
where do i start? i don't get what the question is asking.

C) Calculate [OH-] in the flask after the NaOH has been added.
where do i start? i don't get what the question is asking.

Once you figure out the correct equation for (3), this will make more sense. See hint for (3).