1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Neeeed help

  1. Jan 3, 2008 #1
    Ok I saw this question in another thread and it wasn't answered but said it had previously been answered i've been searching for a while now and can't find it

    Captain Kirk, of the Starship Enterprise, has been told by his superiors that only a chemist can be trusted with the combination to the safe containing the dilithium crystals that power the ship. The combination, which consists of four numbers, is the pH of solution A described below, followed by the pH of solution C. (Example: If the pH of solution A is 3.47 and that of solution C is 8.15, then the combination to the safe is 3-47-8-15.) The chemist must find the combination using only the information below. All solutions are at 25EC.

    Solution A is 50.0 mL of a 0.100 M solution of the weak monoprotic acid HX.

    Solution B is a 0.0500 M solution of the salt NaX. It has a pH of 10.02.

    Solution C is made by adding 15.0 mL of 0.225 M KOH to solution A.

    What is the combination of the safe?

    Hint: You don't need to know the identity of X, except that the X in solution A is the same as in solution B.
  2. jcsd
  3. Jan 4, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    To find the pH of a solution of a weak acid you must know the pKa (or the Ka) which isn't given directly in this problem. You do know that a solution of NaX gives a pH of 10.02. From this you can calculate the Ka. Familiarize yourself with problems of that type before you try to tackle this one.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook