Help with chemistry homework - Acid/Base Equilibrium?

Question

Hi,

I'm having trouble with my AP Chemistry Homework. The problem I'm stuck on is this:

Consider a solution that is made by dissolving 5.2 g of hydrogen iodide (HI) in 2.0 L of pure water at 25oC.
What is the hydronium ion concentration of this solution?

I've figured out the number of moles of HI, .0407, and its initial concentration is .0203 M. Then I constructed an equilibrium table and got the equation Ka = (x^2) / (.0203 - x). From here, though, I don't know how to proceed, as all of the example problems and other problems on the worksheet give the Ka or Kb for the reactant. Can anyone help explain how to solve this problem? Thanks.

Answer 1

Your problem is that you are using a weak acid equation for a strong acid. HI is the strongest of the hydrohalic acids. The question is much easier, since ionisation is 100%. Divide 5.2 by the Mr of HI, and then divide by 2 (for the 2 litres).