# Volume affects temperature change? No. of hydrogen ions?

## 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.

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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 pKa yet?

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

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

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