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In class today Lavelle said that if you have a very small amount of a very weak acid, that changes the answer you get because if the concentration of H30+ is 10^-7 and you get the concentration at the end to be 10^-9, you have to do something different to get the correct answer? What would that be?
Oh so if the x is smaller, than the normal concentration of water then you need to add it to 10^-7 because that will give you the accurate pH, a pH very close to 7. Since 10^-9 is smaller, this means that you would add the x you got to the normal concentration of water.
What Lavelle was talking about is that if you have a very small amount of a weak acid the pH of the mixture remains at 7. This is because adding something like 10^-9 to 10^-7, which you have to do because the H30+ mixture starts off at 10^-7, would give you a number that would remain at 10^-7. Therefore, the amount of weak acid was so small that the pH wouldn't change.
If the concentration of the hydronium ion is smaller then 10^-7, say 10^-9, the pH calculated if you use that will give you 9 - a basic solution. However, you have to keep in mind that there is hydronium ions in the water already and that concentration is 10^-7. So if you add the concentrations of the Hydronium ion from the water and from the weak acid together, you'll see that the concentration is not really affected by the weak acid so the pH is around 7. (To figure out the true value try adding the two concentrations together and negative logging it)
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