Are There NO AP Chem people out there?!? Somebody has to know some of these!!! Come on Science Wizzes!!!?

Question

Teacher gave us these problems and I really! need help with them. I need an answer and how to solve it!!! Thank you and most detailed answer gets best answer tonight!!


4. At zero degrees Celcius the enthalpy of the melting of water is 6.0 kJ times mol to the -1 power. The enthalpy change when 36 grams of water freezes is...?


5. At 100 degrees Celcius the enthalpy of the vaporization of water is 40.7 kJ times mol to the -1 power. The enthalpy change when 36.0 grams of water vapor condenses is...?


6. A 300mL sample of hydrogen chloride at 690 Torr and 20 degrees Celcius is dissolved in 100mL of water. The solution was titrated to the stoichiometric point with 24.8 mL of a sodium hydroxide solution. What is the molar concentration of the NaOH in solution?

Answer 1

If the enthalpy change for melting water is 6.0kj/mole then the change for freezing water should be the same magnitude but opposite sign -6kj/mol (the minus sign implies heat leaves the system). For 36 g of water:

-6.0kj/mole x (36g/18g/mole) = -12.0kj

The change in enthalpy for condensing water is the same magnitude, but opposite sign as that for evaporation, so:

-40.7kj/mol x (36g/18g/mole) = - 81.4kj again, you need to remove heat from the system to condense the water.

For the 3rd part: you need to find out how many moles of HCl you have, then you can solve the problem.

Use the ideal gas equation:

PV=nRT solve for n (moles); n = PV/RT

Get things into proper units:

690 torr = 0.908atm
20 C = 293K
300ml = 0.300L

n = 0.908atm x 0.300L/0.08206Latm/molK x 293K

n = 0.0113mole HCl

The reaction between HCl and NaOH is:

NaOH + HCl ---> NaCl + H2O

the ratio is one HCl to one NaOH so moles HCl = moles NaOH:

24.8ml of the solution contains 0.0113moles of NaOH so:

0.0113mole x (1.00L/0.0248L) = 0.456M