need some help on this

[JuzzLee]

hey, i got a question here, any idea? thanks

100ml of a 0.1 M HS- solution and 100 ml of a 0.2 M HSO3- solution are added separately to two conical flasks containing 100 ml of water. which of the following could be used to accurately determine which acid is the stronger species?

(a) Conductivity
(b) pH
(c) concentration
(d) Equilibarium constant

another question:

The pH of a 10^-9 M solution of HCl is closest to:
(a) 4
(b) 6
(c) 7
(d) 9

i got 9 which is incorrect

[lukeperry91]

hey, i got a question here, any idea? thanks

100ml of a 0.1 M HS- solution and 100 ml of a 0.2 M HSO3- solution are added separately to two conical flasks containing 100 ml of water. which of the following could be used to accurately determine which acid is the stronger species?

(a) Conductivity
(b) pH
(c) concentration
(d) Equilibarium constant

A) In many cases, conductivity is linked directly to the total dissolved solids (T.D.S.), therefore it is not A as the total ammount of solids (atoms) doesn't change.
B) This is a measure of an acids concentration, which is irrespective to its strength
C) "
D) By cancellation this is correct. Also, the strength of an acid determines how readily it will dissociate in water (donate H+ atoms) and the stronger an acid is, the more it will donate, therefore the larger the product:reactant ratio (:. higher K). Therefore the strength of an acid or base correlates to its E_k value.

The pH of a 10^-9 M solution of HCl is closest to:
(a) 4
(b) 6
(c) 7
(d) 9

i got 9 which is incorrect

[H₃O+]=[HCl] as Hydrochloric acid is a strong acid and will completely dissociate.
pH=-log₁₀[H₃O+]
:. pH=-log₁₀[10^-9]
:. pH is 9

No idea how its nine, I'd imagine it would have to be below 7, but only slightly, so I'm gonna say C because of the tiny concentration.

[lukeperry91]

For question two, pH Calculator
         

[quote pH Calculation Results


The contribution of water to the pH has been accounted for due to the low concentration of hydrochloric acid.


The pH of .00000001 M hydrochloric acid is 6.98.

The chemical formula for hydrochloric acid is HCl.

The molecular weight for hydrochloric acid is 36.46097 grams/mole. "
[/quote]
Basically, the effect of the additional formation of hydronium ions causes no significant change to the pH of the solution, I think I remember reading something about this in Heinemann 1. A link to the website I used: http://www.sensorex.com/support/education/pH_calculator.html

The 'flag' word in this question is "...is closest to..." this generally means that the exact answer isn't calculable and you need to refer to knowledge other than the pH formula.

[JuzzLee]

oh i see, thanks man. your getting 50

[ryley]

I think you can do this by hand/calc, to work out the pH, you have to take the negative log of the concentration of H+ in neutral water (10^-7) AND the concentration of H+ from the acid (10^-9), so your final expression for pH should be pH=-log(10^-7+10^-9), which comes out to be 6.99. Its been a while since I did this question, so either I remembered the basis of the procedure, or its a very useful coincidence.

[Mao]

I think you can do this by hand/calc, to work out the pH, you have to take the negative log of the concentration of H+ in neutral water (10^-7) AND the concentration of H+ from the acid (10^-9), so your final expression for pH should be pH=-log(10^-7+10^-9), which comes out to be 6.99. Its been a while since I did this question, so either I remembered the basis of the procedure, or its a very useful coincidence.

That is correct. When adding a very very diluted acid (or base), the products of self-ionisation of water can make a significant contribution. In this case, the addition of a 10^-9 M acid is very insignificant compared to the neutal H+ concentration of 10^-7, hence pH will still be around 7.

The only reason we usually ignore the products of self-ionisation is because the usual concentration of acids we deal with are of a much higher magnitude than 10^-7

[JuzzLee]

thanks guys