Question

Use the data below to calculate ?rH? for the formation of an aqueous solution of H3PO4 from P4O10(s).

compound	?fH?kJmol?1
P4O10(s)	-2984
H3PO4(aq)	-1288.3
H2O(l)	-285.8

Express your answer using one decimal place and include the appropriate units.

answer is -454.4 kjmol -1

but the mestrychemstry assignment says the unit didnt put perobly so plz help ?

Answer 1

We need to find out the change in enthalpy for the following process:
P4O10(s) + 6H2O(l)--> 4H3PO4(s)

Here we will make use of Hess's law of constant heat

?H^o_(reaction) = the sum of the enthalpy (heat) of formation of products - the sum of the enthalpy (heat) of formation of reactants:

?H^o_(reaction) = ?H^o_f(products) - ?H^o_f(reactants)

?H^o_f(H3PO4) = -1288.3 KJ/mol

?H^o_f(P4O10) = -2988.3 KJ/mol

?H^o_f(H2O) = -285.8 KJ/mol

Hence ?H^o_(reaction) = 4 * ?H^o_f(H3PO4) - ?H^o_{f(P4O10) -} 6* ?H^o_f(H2O)

                            = 4 * (-1288.3) - [-2984 - (6* (-285.8)) ]

                            = - 5153.2 + 4698.8

                    ?H^o_(reaction) = - 454.4 KJ/mol

                                         or -454.4 * 10^3 J/mol

Hope this will help !