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December 19, 2014

Homework Help: chemistry

Posted by Heather on Saturday, March 24, 2007 at 1:06pm.

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

pH = - log [H3O+].
PH = 4.7
But it says 20 times larger than pure water and the PH of water is 7 so the PH of this solution has to be 20 x 7 right?

You must have punched a wrong number because (H^+) of 2.0 x 10^-6 gives a pH of 5.7 and not 4.7. Check to make sure that is correct.

And it IS 20 times larger.
Pure water is 1 x 10^-7 M. This solution is 2 x 10^-6 M.
(2E-6/1E-7)= 20.0 on the nose.
20 x 7 isn't correct because the 7 came from the log term. Change pH 7 to (H^+) first to get (H^+) = 1 x 10^-7, THEN divide into 2 x 10^-6.



so divide 7/5.7 = 1.23

My inclination is to say, "No," but I'll ask why?
The 20 times values comes from
2 x 10^-6/1 x 10^-7 = 20.0.

Dividing 7 by 5.7 isn't correct (if you are trying to get the 20.0) because you are dividing a log value by another log value and log values are not directly comparable. For example, look at the following values.
pH.......(H^+)
3.0.....1 x 10^-3
4.0.....1 x 10^-4
5.0.....1 x 10^-5
6.0.....1 x 10^-6
Notice that changing the pH by just 1.0 units, we change the (H^+) by 10.

but im trying to find PH. Is it 5.7?

yes.
If (H^+) = 2E-6, then
pH = -log(H^+)
pH = -log(2E-6)
pH = -(-5.7)
pH = +5.7
Let me know if you have any problems getting that value.

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