How to tell if an Acid/Base is Strong/Weak (1 Viewer)

[melonkitten]

Hi, i have chem trials comming up soon and was wondering if anyone could help me with this

thanks!

 

[minijumbuk]

Erm...

Strong acid/base: Dissociates in water completely to form H+(if acid) or OH- (if base)
Weak acid/base: Dissociates partially (aka incomplete dissociation of acid/base molecules)

 

[timmiitippii]

umm what poster posted b4 me said... but however if its not a qualitative type question but one of those expected to be known knowledge that might pop up in multiple choice and all... u kinda just hav to remember some of the strong acids...
my teacher told us to remember the 5 or 6 and the rest are weak acids and most of the time they just ask questions dealing with hydrochloric or sulfuric anyways so yeh... and these questions generally accompanies pH calculations since its near impossible to calculate pH if they give u weak acid unless u get sum extra info...
but common question i've seen is where they ask u to represent concentrated/dilute strong/weak acid and in these cases u just show how many molecules in sample and how ionised the acid molecules are... but yeh besides these two types... i've never realli seen any other types of questions

 

[melonkitten]

yes, i know this.

what i mean is for example given: NaOH..

How do you determine (approx) the pH of it?

and the pH of a substance is determined by wheter Na and OH are strong/weak acidss/bases right??

 

[Pwnage101]

ok, seriously if u aint gettin it by now, u need 2 do a lot of work

pH = -log[H+]

they will NEVER get u to calculate pH of a weak acid or baser

always when calculating pH u must assume IT FULLY IONISES

plus, pH is not only affected by how strong/weak teh acid is, BUT ITS CONCENTRATION

THESE ACIDS ARE STRONG (MEMORISE THESE!!!!!):

H2SO4
HCL
HNO3

THESE ACIDS ARE WEAK (MEMORISE THESE!!):

H2CO3
CH3COOH
C6H8O7

as T-Pain says - "if u aint got it by now u just aint gettin it!!!!"

 

[minijumbuk]

ok, seriously if u aint gettin it by now, u need 2 do a lot of work

pH = -log[H+]

they will NEVER get u to calculate pH of a weak acid or baser

always when calculating pH u must assume IT FULLY IONISES

plus, pH is not only affected by how strong/weak teh acid is, BUT ITS CONCENTRATION

THESE ACIDS ARE STRONG (MEMORISE THESE!!!!!):

H2SO4
HCL
HNO3

THESE ACIDS ARE WEAK (MEMORISE THESE!!):

H2CO3
CH3COOH
C6H8O7

as T-Pain says - "if u aint got it by now u just aint gettin it!!!!"

Actually, I have seen a few questions that asked to calculate the pH of weak acids/bases. It is possible. Even though it isn't completely ionised and hence there will be some acid molecules that could dissociate, it would not happen, as the ionisation reaches an equilibrium. So there is no problem with calculating pH of a weak acid/base.

yes, i know this.

what i mean is for example given: NaOH..

How do you determine (approx) the pH of it?

and the pH of a substance is determined by wheter Na and OH are strong/weak acidss/bases right??

You must remember that the ONLY thing that changes pH is either H+ concentration or OH- concentration. Na+ has NOTHING to do with the pH.
In your example, you have asked for the pH of NaOH solution. That doesn't make sense, because you did not give the concentration of the solution. If you said it was a 0.1 M solution, then:
pH= -log₁₀[H⁺]
But [H⁺][OH^-] = 1x10^-14
[H⁺]= (1x10^-14 )/ ([OH^-])
Therefore pH= -log₁₀ (1x10^-14 )/ ([OH^-])
= -log₁₀ (1x10^-14 )/ (0.1)
= 13

 

[Azreil]

Actually, I have seen a few questions that asked to calculate the pH of weak acids/bases. It is possible. Even though it isn't completely ionised and hence there will be some acid molecules that could dissociate, it would not happen, as the ionisation reaches an equilibrium. So there is no problem with calculating pH of a weak acid/base.

There was one in my chemistry trial yesterday. It said something along the lines of "A sample of ethanoic acid has a concentration of 0.1155M. Supposing 1% ionisation occurs, calculate the pH of the sample."

If ethanoic acid has a concentration of 0.1155M, the concentration of [H+] is 0.00115.

pH = -log[0.00115] = 2.9

Good luck! =]

 

[katie tully]

Do you guys do conjugate pairs in 11/12?

 

[Azreil]

Yus, we do.

 

[katie tully]

But basically, it might be too late but here's a rundown.

- Strong acids dissociate completely into ions in water.
HA (g or l) + H2O (l) --> H3O+ (aq) + A- (aq)
- Weak acids dissociate very slightly into nions in water
HA (aq) + H2O (l) <--> H3O+ (aq) + A- (aq)

If you make a table of acid-dissociation constants, you will learn acid strengths fairly easily, or you can just look at their formula.

Strong acids basically come in 2 forms; Hydrohalic (HCl, HBr, HI etc)
Oxoacids in which the number of O atoms exceeds the number of ionizable protons by 2 or more such as HNO3, H2SO4, CHlO4, etc.

Weak acids have 4 types;
Hydrohalic acid HF
Those acids in which H is not bonded with O or to halogen such as HCN and H2S
Oxoacids with the number of O atoms equal to or exceeding by one the number of ionizable atoms, like HClO, HNO2, etc

Strong bases are water soluble compounds containing O2- or OH-
M2O or MOH where M = group 1A metal
MO or M(OH)2 where M = Group 2 metal

Weak bases are usually compounds with an electron rich nitrogen atom
Ammonia NH3
Amines such as CH3CH2NH2