A scientist measures the standard enthalpy change for the following reaction to be -20.2 kJ :
Ca(OH)2(aq) + 2 HCl(aq)CaCl2(s) + 2 H2O(l)
Based on this value and the standard enthalpies of formation for the other substances, the standard enthalpy of formation of HCl(aq) is kJ/mol.
Do this to get started.
Look up the standard enthalpy of the other material.
Substitute into the following equation.
dHrxn = (n*dHo products) - (n*dHo reactants)
dH rxn is given in the problem. Substitute the number you look up and solve for the single unknown.
Post your work if you get stuck.