Unit 1+2 Questions

[Lasercookie]

dude, Ive tried doing it already, the thing that doesn't make sense to me though is the fact that I'm ending up with a negative pH and doesn't know how to proceed with that value to find an answer.

pH=-log(concentation of H+ ions)

Are you forgetting that the equation has a negative at the front? You need to put that in to get a positive answer.

(edit: unless this is one of those negative ph situations b^3 is talking about)

[b^3]

Yeh as laserred said (and as I said in my post above)
Answers you should be getting are
1. pH=1
2. pH=3.3
3. pH=12

If you got the -ve of those then you may have forgotten the -ve.

[#1procrastinator]

Find the oxidation numbers of each of the elements in this compound NaClO

I know oxygen is -2 but Na is +1 and Cl is -1. Which one do I change and why? i.e. Na is +3 and Cl is -1 or both Na and Cl are +1

[Lasercookie]

Find the oxidation numbers of each of the elements in this compound NaClO

I know oxygen is -2 but Na is +1 and Cl is -1. Which one do I change and why? i.e. Na is +3 and Cl is -1 or both Na and Cl are +1

The two ions there are Sodium (Na) and Chlorite (ClO).

Chlorite has a charge of 1-



So to show you the working out:
x - 2 = -1
x = -1 + 2
x = +1
So Chlorine must have a charge of +1.



edit: Chlorite is an incorrect name for that ion. It's actually called hypochlorite.
http://en.wikipedia.org/wiki/Chlorite
As you can see though, they all have the overall charge of 1- though.

[dinosaur93]

2H₂PO₄^-    HPO₄^2- + H₃PO₄^-

In the above reaction, the H₂PO₄^- ion acts as:

a. an acid

b. an amphiprotic substance 

c. a reductant

d. an oxidant

Why is the answer b alone? 

[Lasercookie]

2H₂PO₄^-    HPO₄^2- + H₃PO₄^-

In the above reaction, the H₂PO₄^- ion acts as:

a. an acid

b. an amphiprotic substance 

c. a reductant

d. an oxidant

Why is the answer b alone? 

Well it's amphiprotic, so it can't be A.

If you have a reductant, you need something else to be an oxidant. Redox reactions must have both happening simultaneously.

Remember that with acid/base, you have protons being donated/accepted - but electrons are shared. With redox reactions, electrons are being transferred.

[dinosaur93]

12.5g of H₂SO₄ is dissolved in 800 mL of deionised water

Calculate the pH of the solution.

m ( H₂SO₄ ) = 12.5g

N ( H₂SO₄ ) = 800 mL

n ( H₂SO₄ ) =

n ( H₂SO₄ ) =

n ( H₂SO₄ ) = 0.127395 mol

c ( H₂SO₄ ) =

c ( H₂SO₄ ) = = 0.159 M


From this stage on, the solution pathway says:

pH = -log [H₃O⁺ = -log [0.159] = 0.799

but isnt we still need to multiply the concentration by 2 (before solving for pH) because there is 2 H⁺ ions in the given equation?

[dinosaur93]

2H₂PO₄^-    HPO₄^2- + H₃PO₄^-

In the above reaction, the H₂PO₄^- ion acts as:

a. an acid

b. an amphiprotic substance 

c. a reductant

d. an oxidant

Why is the answer b alone? 

Well it's amphiprotic, so it can't be A.

If you have a reductant, you need something else to be an oxidant. Redox reactions must have both happening simultaneously.

Remember that with acid/base, you have protons being donated/accepted - but electrons are shared. With redox reactions, electrons are being transferred.

right! I was thinking of reductant, but in this case since the solution is not mixed or reacted with another solution, therefore it cannot satisfy the redox reaction, is that right? tnx bro!

[#1procrastinator]

Find the oxidation numbers of each of the elements in this compound NaClO

I know oxygen is -2 but Na is +1 and Cl is -1. Which one do I change and why? i.e. Na is +3 and Cl is -1 or both Na and Cl are +1

The two ions there are Sodium (Na) and Chlorite (ClO).

Chlorite has a charge of 1-



So to show you the working out:
x - 2 = -1
x = -1 + 2
x = +1
So Chlorine must have a charge of +1.



edit: Chlorite is an incorrect name for that ion. It's actually called hypochlorite.
http://en.wikipedia.org/wiki/Chlorite
As you can see though, they all have the overall charge of 1- though.

Thanks, teacher treated each ion separately

[dinosaur93]

What is the conjugate base of HPO₄^-?

Is it PO₄^2- or H₂PO₄?

[REBORN]

first.

[dinosaur93]

I knew it!

because the solution pathway that I had says H₂PO₄!

tnx heaps bro!