A 90.0-mL sample of 1.00 M NaOH is mixed with 45.0 mL of 1.00 M H_2S0_4 in a lar

Question

A 90.0-mL sample of 1.00 M NaOH is mixed with 45.0 mL of 1.00 M H_2S0_4 in a large Styrofoam coffee cup; the cup is fitted with a lid through which passes a calibrated thermometer. The temperature of each solution before mixing is 21.1 degree C. After adding the NaOH solution to the coffee cup and stirring the mixed solutions with the thermometer, the maximum temperature measured is 32.1degree C. Assume that the density of the mixed solutions is 1.00 g/ml, that the specific heat of the mixed solutions is 4.18 J/(g middot degree C), and that no heat is lost to the surroundings. Write a balanced chemical equation for the reaction that takes place in the Styrofoam cup. Is any NaOH or H_2SO_4 left in the Styrofoam cup when the reaction is over? Calculate the enthalpy change per mole of H_2SO_4 in this reaction.

Explanation / Answer

H2SO4 + 2NaOH --> Na2SO4 + 2H2O

mmol of acid = MV = 1*45= 45mmol

mmol of base= MV = 1*90 = 90mmol of base

since ratio is 2:1, then 90:45 is valid; expect no excess material

Then

Hrxn = -Q/n

n = 45 mmol = 45*!0^-3

Q = m*Cp*(Tf-Ti)

m = m1+m2 = 90+45 = 135

Q = 135*4.184*(32.1-21.1) = 6213.24

Hrxn = -6213.24/(45*10^-3) = -138072 kJ/mol

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