Calculate the heat of formation (or deltaHf) of P_4O_10 (s) under these conditions

Question

When 10.00 g of phosphorus is burned in O_2(g) to form P_4O_10(s), enough heat is generated to raise the temperature of 2950 g of water from 18.0 degrees C to 38.0 degrees C? Answer in kJ/mol.

Please Help! and explain! thank you!

Answer 1

Use

heat = mass of water (g) x change in temperature (degree) x specific heat of water (4.184 J g-1 degree-1)

That is the amount of heat generated from 10.0 g P.

Find the mass of phosphorus in 1 mole P4O10, and scale up accordingly.

Since the reaction is the formation of the compound from its elements, the heat of reaction is the same thing as the heat of formation.

Remember to convert to kJ/mol, and round down to the correct number of significant figures if your instructor is worried about that kind of thing.

Over to you.  Good luck.

Answer 2

38.0C - 18.0C = 20.0degC difference

The heat capacity of H2O is 4.184 J/g-degC

P4 + 5O2 ===> P4O10

Atomic weights: P=31  O=16  P4=124  P4O10=284

10gP4 x 1molP4/124gP4 x 1molP4O10/1molP4 = 0.08064 mole P4O10 formed

2950gH2O x 20.0degC x 4.184J/g-degC = 246,856 J = 246.8 kJ to four significant figures

246.8kJ/0.08064molP4O10 = 3060 kJ/mole to four significant figures