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[~*HSC 4 life*~]

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some q's out of conquering chem which i am too dumb to understand lol

1. which list contains only acidic oxides?
a)CO, CO2, B2O3, SO3
b) Cl20, P2O5, NO2, SO2
c) P203, NO, SO2, Cl2O
d) N2O, NO2, CO2, SO3

2. The volume oc CO2 measured at 0 degrees and 1 atm pressure that can be absorbed by 0.30 mol KOH is


how u meant to work that out??? ^^

q3. the pH of a solution of sulfuric acid was 3.4. This means that in thats olution, the sulfate ion conc in moles per litre was...

And for a part c question, you have to calculate a weak acids degree of ionisation....how do you do that?


cheers!

[Elliot III]

I might be able to help you!
Well most metal oxides are bases and non metal oxides are acidic. In this particular question, seeming they are all non- metals, I guess what they are looking for is neutral oxides.
CO N2O NO are neutral oxides. So the answer should be b)

With question 3 I'm not sure if you would still use the ph to concentration method where you just enter in SHIFT log -pH. This should give you a concentration of 0.0004 (0.000398) mol/L?

I hope that helps!

[Constip8edSkunk]

For Weak Acids HA:
Ka = ( [H3O+] [A-] ) / [HA]
Ka being the equilibtium constant of HA usualy @ SLC

if let [H3O+] = x then [A-] =x and the original concentration of [HA] decreases by x. then solve the quadratic

btw this isnt in the course

[Xayma]

3. pH=-log [H+] therefore
3.4=-log[H+]
10^-3.4=[H+]
[H+]= 3.98 x10^-4
2[H+]=[SO4^2-]
therefore sulfate concentration=1.99x10^-4mol/L

[~*HSC 4 life*~]

Originally posted by Elliot III
I might be able to help you!
Well most metal oxides are bases and non metal oxides are acidic. In this particular question, seeming they are all non- metals, I guess what they are looking for is neutral oxides.
CO N2O NO are neutral oxides. So the answer should be b)

ur rigjt! what is a neutral oxide? and why are they neutral?

Originally posted by Xayma
3. pH=-log [H+] therefore
3.4=-log[H+]
10^-3.4=[H+]
[H+]= 3.98 x10^-4
2[H+]=[SO4^2-]
therefore sulfate concentration=1.99x10^-4mol/L

your ans is right, i dont get the last 2 steps

[~*HSC 4 life*~]

Originally posted by Constip8edSkunk
For Weak Acids HA:
Ka = ( [H3O+] [A-] ) / [HA]
Ka being the equilibtium constant of HA usualy @ SLC

if let [H3O+] = x then [A-] =x and the original concentration of [HA] decreases by x. then solve the quadratic

btw this isnt in the course

ive got no idea what the hell that means- is it usefful?

[Constip8edSkunk]

not 4 the HSC as its outside the syllabus

Originally posted by ~*HSC 4 life*~
ur rigjt! what is a neutral oxide? and why are they neutral?

EDIT: yeah see what affinity said

your ans is right, i dont get the last 2 steps

10^-3.4 = 3.98 x10^-4 (putting it in scientific notation)
and since H2SO4 is diprotic, ie with 2 hydrogen atoms, there would be 1/2 as many sulfate ions in concentration(even though in reality i dun think they completely ionise twice as such) therefor 3.98 x10^-4 is divided by 2

[Affinity]

neutral oxides don't react as acids nor bases. Neutral is different from amphoteric (such as ZnO)

[CM_Tutor]

Question 1 is B. As Elliott III pointed out, the non-metal oxides CO, N2O and NO are neutral, which rules out A, C and D. Furthermore, all the oxides in B react with water to form recognisable acids:

Cl2O + H2O ---> 2HOCl (hypochlorous acid)

P2O5 + 3H2O ---> 2H3PO4 (phosporic acid)

2NO2 + H2O ---> HNO2 + HNO3 (equimolar mixture of nitrous acid and nitric acid)

SO2 + H2O ---> H2SO3 (sulfurous acid)

Question 2: "The volume of CO2 measured at 0 degrees and 1 atm pressure that can be absorbed by 0.30 mol KOH is"

depends on the equation you use.

If you use: KOH + CO2 ---> KHCO3
then 0.30 mol of KOH will absorb 0.30 mol of CO2, or about 6.7 L of the gas at STP.

If you use: 2KOH + CO2 ---> K2CO3 + H2O
then 0.30 mol of KOH will absorb 0.30 / 2 = 0.15 mol of CO2, or about 3.4 L of the gas at STP.

Re: "you have to calculate a weak acids degree of ionisation....how do you do that?"

The % ionisation of a weak acid, HA, is [H+] / orig [HA], expressed as a percentage.

For example, the pH of 0.100 M acetic acid is 2.88 at 25 �C. Thus, its percentage ionisation is:

pH = 2.88 <-----> [H+] = 10^(-2.88) = 1.3182 ... x 10^(-3) M

So, % ionisation = (1.3182 ... x 10^(-3) / 0.100 ) x 100 / 1 = 1.32 %

Similarly, since 0.100 M formic acid is 4.27 % ionised at 25 �C, its pH is

[H+] = 4.27 % x 0.100 M = 0.00427 M

So, pH = -log 0.00427 = 2.37

Questions on percentage ionisation are asked in past papers. The concept is related to the Ka stuff that Constip8tedSkunk mentioned, but that isn't in the HSC syllabus. (In fact, Ka is the correct way to deal with weak acids, % ionisation is a used by the HSC so as to avoid the slightly more complicated maths.)

NOTE: % IONISATION IS A CONCENTRATION DEPENDANT VALUE, and so 0.200 M acetic acid will have a different % ionisation than 0.100 M acetic acid. This should ALWAYS be borne in mind when using it, especially if you are trying to compare the relative strengths of weak acids.

[Xayma]

Originally posted by Constip8edSkunk
and since H2SO4 is diprotic, ie with 2 hydrogen atoms, there would be 1/2 as many sulfate ions in concentration(even though in reality i dun think they completely ionise twice as such) therefor 3.98 x10^-4 is divided by 2

No they don't completly dissosciate. HSO4- is a weak acid so forms an equilibrium but I think as far as the HSC syllabus is concerned if it asked a question like that it means completly ionised.