If a solution contains 0.01 mol HCl in 10L of solution, what's its molarity? Its H+ ion concentration? Its pH?

Question

I'm having trouble figuring out how to do this problem. I figured out that the molarity is 0.001 M HCl, but I have no clue how to find its H+ ion concentration.... or its pH.

I don't want just the answers, I need to know how to do the problem so that I can do additional problems as well.

Thanks!

The next two questions also have me stumped--I missed class because I have the flu. :(

They are:

What relationship exists between H+ ion concentration and OH- ion concentration? What is the OH- concentration in an aqueous solution where the H+ concentration is 10^-4 M?

Answer 1

Molarity is defined as number of moles in 1 liter.
You have 0.01 mol and 10 liters
M = 0.01 / 10 mol/liter - 0.001 mol/liter = 0.001 M. That's it. You did it.

Now pH = - log[H+].

If you write the equation
HCl ---> H+ + Cl-
This means that you have the same molarity for HCl and H(+). For both of them the coefficient in the chemical equation is equal to 1. For other problems the coefficients may be different so always look at the balanced equation.

so M(HCl) = M(H+) = 0.001.

Putting this value into the pH equation:

pH = - log 0.001 = 3

pH + pOH = 14.
Calculate the pH according to the pH definition and then use the above equation to find the pOH.

Answer 2

pH = -log[H+]

This means that you take the opposite of the log of the concentration of HCl (which is equal to [H+]). Your concentration is 10^-3, therefore, pH=3.

Also, remember that [H+] + [OH-] = 14. ALWAYS. So, if your [H+]=4, then [OH-]=10.