Water in contact with air is acidic, rather than neutral, due to
dissolved carbon dioxide. Water in equilibrium with the air contains
4.4 x 10-5 % CO2. The resulting carbonic acid, H2CO3, gives the
solution a hydrogen concentration of 2.0 x 10-6 M, about 20 times
larger than that of pure water. Calculate the pH of the solution.

Hello teatea!
The pH is calculated as 

pH = -log(H+), 

where H+ is the hydrogen ion concentration, measured in mol L-1 (M).
Since we already know the concentration for this solution, we can
simply plug this number into the formula for pH. Therefore, the pH of
the solution is:

pH = -log(2.0x10-6) = 5.69

which is, of course, less than 7 because it is an acid. Check the
following page for more information on how pH is calculated.

Defining pH
http://www.ausetute.com.au/phscale.html


Google search strategy
calculate ph
://www.google.com/search?q=calculate+ph&hl=en&lr=&ie=UTF-8&oe=UTF-8&start=10&sa=N


I hope this helps. If you have any doubt regarding my answer, please
request a clarification before rating it. Otherwise, I await your
rating and final comments.


Best wishes!
elmarto