In: Chemistry
Determine pH for a solution containing 0.7 M HBr and 1.4 M HC3H5O2.
A strong acid is defined as one that dissociates completely in
water (see the Related Questions link to the left for a list of the
strong acids). That means for every mole of acid added, one mole of
free H+ (or equivalently, H3O+) is present in the solution. The pH
of solution is defined this way:
pH = -log10 [H3O+]
or in English, the pH is equal to the negative logarithm (in base
10) of the concentration of H3O+ in the solution (the concentration
must be in units of Molarity (M), which is moles per liter).
So to find the pH of a strong acid solution all you need to know is
the concentration of the solution. Let me give a couple of
examples.
pH = -log (0.01) = 2
Well, this is special because sulfuric acid is a (quite) strong
diprotic acid in diluted solutions (below 0.1 M).
Therefore the concentration of H3O+ is twice the molarity of
H2SO4 because this holds 2
protons (H+) per mole.
So in a 0.007 M solution of sulfuric acid, H2SO4, the concentration
of H3O+ is:
0.007