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Calculate the enthalpy change when 50 mL of 0.01 M Ca(OH)2 solution reacts with 25 mL of 0.01 M HCl solution. The standard enthalpy of neutralisation of a strong acid with a strong base is -57.1 kJ/mol. (Assume Ca(OH)2 is a strong base.)

1. -14.275 J
2. -14.275 kJ
3. -14.275 * 10³ kJ
4. -14.375 * 10³ J

Correct answer: -14.275 J

Solution

Ca(OH)2 is diprotic base; it gives 2 OH- per formula unit. Moles of OH- = 2 * (0.050 L * 0.01 mol/L) = 1*10⁻³ mol. Moles of H+ from HCl = 0.025 L * 0.01 mol/L = 2.5*10⁻⁴ mol. H+ is limiting: only 2.5*10⁻⁴ mol of neutralisation occurs. delta_H = 2.5*10⁻⁴ mol * (-57100 J/mol) = -14.275 J.

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