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Homework Help: Volume affects temperature change? No. of hydrogen ions?

  1. Jul 23, 2011 #1
    1. The problem statement, all variables and given/known data

    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.
  2. jcsd
  3. Jul 23, 2011 #2
    The enthalpy is doubled -- you're right about that. What's the relationship between enthalpy and temperature change?

    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?
  4. Jul 24, 2011 #3
    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? -.-
  5. Aug 8, 2011 #4
    So, you are saying that for question a), I'm correct?
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