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: What percentage is in the form of neutral molecules?

  1. Apr 18, 2010 #1
    If two aspirin tablets, each having a mass of 325mg, are dissolved in a full stomach whose volume is 1L and whose pH is 2, what percent of the aspirin is in the form of neutral molecules? (At body temperature, the Ka for aspirin = 3*10^-5)

    My Work:
    I know that the molecular formula for aspirin is C9H8O4. This means that aspirin has a molecular weight of 180 g/mol. 650 mg of aspirin is equal to 3.6*10^-3 moles of aspirin.
    I also know that the stomach produces gastric acid which is primarily hydrochloric acid. Because the stomach has a pH of 2, then [H+] is 0.01M. Because HCl is a strong acid, we can assume that it completely ionizes and so [H+] is equal to the initial concentration of HCl. I also am pretty sure that I am going to need to use the formula for percent ionization which is ([H+]/initial concentration) x 100. Since ionization is the conversion of a neutral atom into a charged ion, I would have to subtract the percent ionized from 100% in order to get the percent of neutral molecules.

    My train of thought has led me to believe that my equation will be [H+]/initial concentration of HCl x 100, but the [H+] is not just concentration H+ given by HCl - I also have to take into account the aspirin to get the right [H+]. I am going to need to set something equal to the Ka of aspirin (presumably x^2/3.6*10^-3) which gives me x = [H+] = [C9H7O4-] = 3.29*10^-4.

    So what I am wondering is, did I do this work correctly? If so, how do I proceed from here? Just put the x value/initial concentration of HCl? If I did this incorrectly, where did I go wrong and what do I need to do to fix it?
  2. jcsd
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted