Calculate the enthalpy change of formation for the following reactions using Hess's law, and the enthalpies of formation given in Figure 4:

b) MgO +2HCl --> MgCl2 +H2O
c)NaOH+ HCl -->NaCl +H20

Figure4: the given enthalpies of formation compound values are:
Mg(OH)2 is 925kj mol-1

H2O is -286 kj mol-1
NaOH is -169 kj mol-1
MgO is -602 kjmol-1
HCl is -167 kjmol-1
MgCl2 is -641 kj mol-1
NaCl is -107 kjmol-1

This question isn't worded very well in my opinion but I think you want just the enthalpy for the reaction given. Those are

dHrxn = (n*dH products) - (n*dH reactants)
dHrxn = (1*MgCl2 + 1*H2O) - (1*MgO + 2*HCl)
dHrxn = [1*(-641) + 1*(-286)] - [1*(-602) + 2*(-167)] = ?

To calculate the enthalpy change of formation for the given reactions using Hess's Law, we need to use the enthalpies of formation of the compounds involved in the reactions.

a) MgO + 2HCl --> MgCl2 + H2O

The enthalpy change of formation for this reaction can be calculated using the following steps:

1. Write the balanced chemical equation for the reaction:
MgO + 2HCl --> MgCl2 + H2O

2. Determine the enthalpy change of formation for each compound involved in the reaction. The enthalpy of formation is given in Figure 4:

Enthalpy of formation for MgO: -602 kJ/mol
Enthalpy of formation for HCl: -167 kJ/mol
Enthalpy of formation for MgCl2: -641 kJ/mol
Enthalpy of formation for H2O: -286 kJ/mol

3. Apply Hess's Law, which states that the total enthalpy change for a reaction is equal to the sum of the enthalpy changes for the individual reactions that make up the overall reaction.

The overall reaction can be broken down into the following steps:

a) Mg(s) + Cl2(g) --> MgCl2(s) (formation of MgCl2)
b) MgO(s) + HCl(g) --> MgCl2(s) + H2O(l)

The enthalpy changes for these steps can be calculated as follows:

Step a):
Mg(s) + Cl2(g) --> MgCl2(s)

The enthalpy change of this step can be calculated using the enthalpy of formation of MgCl2:
Enthalpy change for step a) = -641 kJ/mol

Step b):
MgO(s) + HCl(g) --> MgCl2(s) + H2O(l)

The enthalpy change of this step can be calculated by subtracting the enthalpy of formation of the reactants from the enthalpy of formation of the products:
Enthalpy change for step b) = [Enthalpy of formation of MgCl2 + Enthalpy of formation of H2O] - [Enthalpy of formation of MgO + Enthalpy of formation of HCl]

Substitute the given values:
Enthalpy change for step b) = [(-641 kJ/mol) + (-286 kJ/mol)] - [(-602 kJ/mol) + (-167 kJ/mol)]

Calculate the enthalpy change for step b) to get the enthalpy change of formation for the reaction.

c) NaOH + HCl --> NaCl + H2O

Follow the same steps as above, using the given enthalpies of formation:

Enthalpy change for step a) = -107 kJ/mol
Enthalpy change for step b) = [(Enthalpy of formation of NaCl + Enthalpy of formation of H2O) - (Enthalpy of formation of NaOH + Enthalpy of formation of HCl)]

Substitute the given values into the equation and calculate the enthalpy change for step b) to get the enthalpy change of formation for the reaction.