A student mixes a 10.0 mL sample of 1.0 M NaOH (aq) with a 10.0 mL sample of 1.0

Question

A student mixes a 10.0 mL sample of 1.0 M NaOH (aq) with a 10.0 mL sample of 1.0 M HCl(aq) in a polystyrene container. The temperature of the solutions before mixing was 20.0 degree C. If the final temperature of the mixture is 26.0 degree C, what is the experimental value of Delta H degree_rxn? (Assume that the solution mixture has a specific heat of 4.2 J/g middot C and a density of 1.0 g/mL.). (A) -50. kJ/mol_rxn (B) -25 kJ/mol_rxn (C) -5.0 times 10^4 kJ mol_rxn (D) -5.0 times 10^2 kJ/mol_rxn

Explanation / Answer

HRxn = Qreaction / mol of reaction

mol of rxn = M*V = 10*1 = 10 mmol = 10*10^-3 mol

Qwater = m*C*(Tf-Ti)

Qwater = (10+10)(4.184)(26-20) = 502.08 J

Qrxn = -Qwater = -502.08 J

HRxn = -502.08/(10*10^-3) = -50,208 J = -50.21 kJ/mol

Best answer is A

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