Volume affects temperature change? No. of hydrogen ions?

[Kyoma]

Homework Statement

a) 100 cm³ of 0.1 mol/dm³ of hydrochloric acid was added to 100 cm³ of 0.1 mol/dm³ of sodium hydroxide and a rise in temperature of 4 degrees was observed. If 200 cm³ of 0.1 mol/dm³ of hydrochloric acid and 200 cm³ of 0.1 mol/dm³ of sodium hydroxide were used, the rise in temperature would be about?

b) Which solution contains the greatest concentration of hydrogen ions?
1. 2 mol/dm³ of sulfuric acid
2. 5 mol/dm³ of ethanoic acid
3. 2 mol/dm³ of sulfurous acid
4. 2 mol/dm³ of phosphoric acid

2. The attempt at a solution

a) I put 8 degrees as the answer since there are twice as many moles for each solution, but I was wrong. The correct answer is 4 degrees. Does this mean that enthalpy change is unaffected by the number of moles of the reactants?

b) I put 4 as the answer since its molecular formula is H₃PO₄, thus there are 3 moles of hydrogen ions in one molecule of the acid and multiplying it by the concentration gives me 6 moles of hydrogen ions, the highest, but I was wrong again. The answer is 1. I have no idea.

 

[pmsrw3]

a) I put 8 degrees as the answer since there are twice as many moles for each solution, but I was wrong. The correct answer is 4 degrees. Does this mean that enthalpy change is unaffected by the number of moles of the reactants?

The enthalpy is doubled -- you're right about that. What's the relationship between enthalpy and temperature change?

b) I put 4 as the answer since its molecular formula is H₃PO₄, thus there are 3 moles of hydrogen ions in one molecule of the acid and multiplying it by the concentration gives me 6 moles of hydrogen ions, the highest, but I was wrong again. The answer is 1. I have no idea.

Phosphoric acid can give up as many three protons. But it is not very willing to do that -- in an acidic solution, it will hang on to some of the three. This can be calculated. Have you studied pK_a yet?

 

[Kyoma]

So, my answer for question one is correct? But the correct answer is 4 degrees?

And, no, I have not learned pKa yet... Is it possible if you can give a brief introduction on pKa? -.-

 

[Kyoma]

So, you are saying that for question a), I'm correct?