A2 AQA Confusing find pH question

Applicant75235

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Reply 1

KaylaB

Original post by koolgurl14

When water is added, does the solution become stronger or weaker?

Reply 2

ghostmarston

First calculate the moles of Hcl= 0.154*10/1000=1.54x10-3
then calculate the new concentration of Hcl after adding water= 1.54x10-3/(990+10)*1000=1.54x10-3
then caluclate pH= -log(1.54x10-3)=2.81

(edited 8 years ago)

Reply 3

Applicant75235

Original post by ghostmarston

First calculate the moles of Hcl= 0.154*10/1000=1.54x10-3
then calculate the new concentration of Hcl after adding water= 1.54x10-3/(990+10)*1000=1.54x10-3
then caluclate pH= -log(1.54x10-3)=2.81

dividing with 0.1 deffo wont give u the same number?

Reply 4

Applicant75235

Original post by KaylaB

When water is added, does the solution become stronger or weaker?

Weaker?

Reply 5

ghostmarston

where did you get 0.1 from?

Reply 6

B_9710

Original post by KaylaB

When water is added, does the solution become stronger or weaker?

I know what you mean but that question doesn't make much sense.

Reply 7

KaylaB

Original post by koolgurl14

Weaker?

Exactly, as the answers has been posted above, just remember that the general way to do these questions is to calculate the amount of moles of the acid in the beginning, then calculate the concentration of the acid for the total volume, then find the pH as you would normally :h:

Reply 8

KaylaB

Original post by B_9710

I know what you mean but that question doesn't make much sense.

Care to elaborate?

Reply 9

B_9710

You know the concentration of H+ in water at 298k is around

10^{-7} \text{M}

, using n=cv you can work out the moles of H+ in the water.
Then you want to work out the moles of H+ added to the solution.
Then use pH

= -\log[\text{H}^+]

(edited 8 years ago)

Reply 10

B_9710

Original post by KaylaB

Care to elaborate?

Well depending on what you add to a solution, the solution could get more strongly acidic, more strongly alkaline, or more weakly acidic or alkaline.

Reply 11

KaylaB

Original post by B_9710

Well depending on what you add to a solution, the solution could get more strongly acidic, more strongly alkaline, or more weakly acidic or alkaline.

I said about adding water a neutral substance, which can be used for dilution

Reply 12

SubwayLover1

Use the formula :

H+ new =[H+] old x Old Volume / New Volume

Reply 13

B_9710

Original post by KaylaB

I said about adding water a neutral substance, which can be used for dilution

That's not what the question said though. But yeah cool I'm not saying your wrong though.

Reply 14

KaylaB

Original post by B_9710

That's not what the question said though. But yeah cool I'm not saying your wrong though.

It literally says in the question that the HCl is added to 990cm^3 of water

Reply 15

B_9710

Original post by KaylaB

It literally says in the question that the HCl is added to 990cm^3 of water

So does adding acid to water make the solution stronger?

Reply 16

KaylaB

Original post by B_9710

So does adding acid to water make the solution stronger?

Well there isn't really a concentration of water, so adding some acid to water would still just make a dilute acid

Reply 17

B_9710

Original post by KaylaB

Well there isn't really a concentration of water, so adding some acid to water would still just make a dilute acid

Yeah I know, this is why what you said doesn't make much sense, the acid gets more dilute, but you can't say it gets weaker. pH does not really tell you how strong an acid is unless you're comparing 2 acids with same conc.