who is right?

Question

we had our chem final and my friend and i were disputing an answer.

the question was about a x mL of NH3 of .100 M being titrated with x mL of .100 M HCl

It said:

a- if you had an unknown amt. of water to the HCl, will the resulting pH be too high or too low and why?

I said you would be diluting the HCl so the pH would be higher than it normally would be. My friend said adding water results in more production of H+ which would make the pH lower. Who is right? Or neither?

Then the second part was adding water to the NH3 and the same concept, but I thought the change would be negigible since it is a weak base. My friend has the same logic as before but thought higher and I thought lower.

The last part was using phenophthalein as an indicator and we agreed here. We said that it would be higher since the range for this indicator is 8-9.6....but we may both be wrong??

Answer 1

If you dilute HCl with water, you are changing the concentration. Remember what pH means: -log[H+]

Therefore, if you have a 0.1 M solution, your pH will be 1. But if you dilute 10-fold such that your concentration is now 0.01M, the pH will be 2. If you dilute another 10-fold such that your concentration is 0.001M, the pH will be 3. In other words, diluting will cause the pH to increase (become more basic). So, you are correct.

Ammonia, or NH3, has a pH around 11. But this is due to this reaction:

NH3(aq) + H2O(l) ----> NH4+(aq) + OH-(aq)

In water, ammonia will form NH4OH, which is a weak alkali base. Hence you are starting out with a basic solution.

Adding water to this will continue to dilute the ammonia. Hence, the pH will go down (become more acidic). Therefore, it appears your friend is correct.

I'm not sure I understand your indicator question as it is written. You wrote "it would be higher". What would be higher? If you meant could phenolphthalien be used as an indicator with the ammoia in water solution, then yes. Above pH 9, this indicator turns pink. Below that, it is colorless. If you start out with a more concentrated base, the indicator will turn it pink. As you dilute with water, it will turn clear.