Results are out! Find what you need...fast. Get quick advice or join the chat
Hey there Sign in to join this conversationNew here? Join for free

Chem calculation help please!

Announcements Posted on
    • Thread Starter
    • 1 follower
    Offline

    ReputationRep:
    Came across this past paper Q and I really don't know how to go about doing it!
    Mark scheme is v. concise, but the answer is 19.4g

    How would you get this ans though?

    I've attached the Q

    would appreciate it soooo much!
    Attached Thumbnails
    Click image for larger version. 

Name:	chem que.jpg 
Views:	16 
Size:	82.3 KB 
ID:	154170  
    • 16 followers
    Offline

    ReputationRep:
    (Original post by Jasmine_777)
    Came across this past paper Q and I really don't know how to go about doing it!
    Mark scheme is v. concise, but the answer is 19.4g

    How would you get this ans though?

    I've attached the Q

    would appreciate it soooo much!
    In the previous must they must have told you which one is the acid and which one is the salt!
    well if they didn't then we must work it out..this is not OCR is it!

    EDIT:I got as far as finding the ratio of salt/acid..then I need a volume which is not given..
    • 21 followers
    Offline

    ReputationRep:
    (Original post by Jasmine_777)
    Came across this past paper Q and I really don't know how to go about doing it!
    Mark scheme is v. concise, but the answer is 19.4g
    The weak acid equilibrium:

    Ka = [H+][A-]/[HA]

    where [HA] is the acid, [A-] is its conjugate base.

    You are told that pH = 7, therefore [H+] = 1 x 10-7

    Ka = 6.2 x 10-8

    Both compounds are in the same solution so volumes cancel out. Hence:

    6.2 x 10-8/1 x 10-7 = 0.1/[HA]

    [HA] = 0.1613

    Relative mass = 120

    Therefore mass needed in 1dm3 = 0.1613 * 120 = 19.35g

Reply

Submit reply

Register

Thanks for posting! You just need to create an account in order to submit the post
  1. this can't be left blank
    that username has been taken, please choose another Forgotten your password?
  2. this can't be left blank
    this email is already registered. Forgotten your password?
  3. this can't be left blank

    6 characters or longer with both numbers and letters is safer

  4. this can't be left empty
    your full birthday is required
  1. By joining you agree to our Ts and Cs, privacy policy and site rules

  2. Slide to join now Processing…

Updated: June 5, 2012
New on TSR

What do you think of the BBC?

UK Parliament wants to hear your views

Article updates
Reputation gems:
You get these gems as you gain rep from other members for making good contributions and giving helpful advice.