Question

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27.) C 28.) B 29.) B

27) The value of AHº for the reaction below is +128.1 kJ: CH3OH (1) CO (g) + 2H2 (g) How much heat is consumed when 87.1 g of hydrogen gas is formed? A) -2.76x103 kJ B) -128.1 kJ C) 2.76 x103 kJ D) -5.52 x103 kJ E) 5.52 x103 kJ 28) How many joules of heat are absorbed when the temperature of a 13.9 g sample of CaCO3 (s) increases from 21.7 °C to 33.3 °C? Specific heat of calcium carbonate is 0.82 J/g-K. A) 9.5 J B) 130J C) 0.68 J D) -0.68 J E) S-130 J 29) When 0.7521 g of benzoic acid was burned in a calorimeter containing 1,000. g of water, a temperature rise of 3.60°C was observed. What is the heat capacity of the bomb calorimeter, excluding the water? The heat of combustion of benzoic acid is -26.42 kJ/g. A) 752.1 kJ/°C B) 1.34 kJ/°C C) 4.18 kJ/°C D) 15.87 kJ/°C E) 5.52 kJ/°C

Answer 1

27)

Molar mass of H2 = 2.016 g/mol

mass(H2)= 87.1 g

use:

number of mol of H2,

n = mass of H2/molar mass of H2

=(87.1 g)/(2.016 g/mol)

= 43.2 mol

Since Δ H is positive, heat is absorbed

when 2 mol of H2 produced, heat absorbed = 128.1 KJ

So,

for 43.2 mol of H2, heat absorbed = 43.2*128.1/2 KJ

= 2.76*10^3 KJ

Answer: C

28)

Given:

m = 13.9 g

C = 0.82 J/g.oC

Ti = 21.7 oC

Tf = 33.3 oC

use:

Q = m*C*(Tf-Ti)

Q = 13.9*0.82*(33.3-21.7)

Q = 130 J

Answer: B

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