Hey! Sign in to get help with your study questionsNew here? Join for free to post

Determining unknown quantities question

Announcements Posted on
Would YOU be put off a uni with a high crime rate? First 50 to have their say get a £5 Amazon voucher! 27-10-2016
    • Thread Starter

    Anyone know the answer to this? It was in one of the multiple choice tests we were given online. I got the answer wrong and was given the correct answer (3) but not any of the working out so I don't know how to reach the answer.

    Read the information below about an acid represented by the formula HxY.

    •One mole of the acid represented by HxY ionises to produce x mol of hydrogen ions.
    •The molar mass of the acid is 192 g mol−1.
    •2.4 g of the acid was dissolved in water and the volume made up to 250 cm3.
    •20 cm3 of this solution was neutralised by exactly 25.0 cm3 of a 0.12 mol dm−3 solution of NaOH(aq).

    What is the value of x?



    Since the Mr is 192g, you can find the number of moles in your 250 cm^3 solution by doing 2.4/192. This gives you 0.0125 moles.

    Since you are only using 20cm^3 of your solution, you multiply 0.0125 by 20/250 to find the number of moles present in your 20cm^3 solution. This comes to 1 x 10^-3 moles

    Then workout the no. moles of NaOH with vol x conc, which gives you 3 x 10^-3 moles. You then have a molar ratio of HxY: NaOH, 1:3.

    Therefore your reaction must be HxY + 3NaOH --> 3NaY + 3H20. Hence, your x value must be 3.

    Hope this helps
    • Thread Starter

    Thanks, that's great! I knew I was missing a step, turns out I forgot to multiply to find the amount of moles in 20cm^3 rather than 250. Thanks again
Write a reply…


Submit reply


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. Oops, you need to agree to our Ts&Cs to register
  2. Slide to join now Processing…

Updated: April 6, 2016
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

Would you rather be able to

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

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