Question

# Sodium hydroxide (NaOH) has a lattice energy of -887 kJ/mol and a heat of hydration of...

Sodium hydroxide (NaOH) has a lattice energy of -887 kJ/mol and a heat of hydration of -932 kJ/mol. How much solution could be heated to boiling by the heat evolved by the dissolution of 28.0 g of NaOH? (For the solution, assume a heat capacity of 4.0 J/g⋅∘C, an initial temperature of 25.0 ∘C, a boiling point of 100.0 ∘C, and a density of 1.05 g/mL.)

We need to consider

a) Heat evolved due to dissolution of NaOH

b) Heat required to boil the solution

Let us calculate heat evolved in dissolution of NaOH

Heat of dissolution = Heat of hydration - Lattice energy = -932 - (-887) = -45 KJ / moles

The amount of NaOH dissolved = 28 grams

Molecular weight of NaOH = 40 g / mole

So Heat of dissolution = -45 X 28 / 40 = 31.5 KJ

Now let us calculate heat required

Heat requried = Mass X Heat capacity of solution X change in temprature

Heat utilized = Heat given

31.5 X 10^3 Joules = Mass X 4 X 75

Mass = 105 grams

Density = 1.05 g / mL

so volume which will be heated = Mass / Density = 105 / 1.05 = 100mL

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