9. (16pts) A sample of an unknown metal was reacted with 150.0g of hydrochloric

Question

9. (16pts) A sample of an unknown metal was reacted with 150.0g of hydrochloric acid in a calorimeter. If 3.457 g sample of the metal caused the temperature of the calorimeter and its contents to rise from 22.4 to 72.1, calculate the heat of reaction per gram for the metal. The acid solution has a specific heat of 4.168 J/gk, and the calorimeter has a heat capacity of 39.2 J/K. a. The hydrogen gas from the experiment above was collected in a 785 mL container at 30.0°C and 731 torr pressure, how many moles of hydrogen were collected? b. What is the molar mass of the metal if previous experiments demonstrated the metal to form a chloride salt of the formula MCl,? Be sure to show the balanced equation you used to determine the mols of metal reacted. c. d. Finally, calculate the molar heat of reaction of the metal.

Explanation / Answer

a)

m = 3.457 g

dT = 72.1-22.4 = 49.7°C

Qsoln = m*C*dT = 150*4.184*49.7

Qcal = Ccal*dT = 39.2*49.7

Qrxn = -31191.72 - 1948.24 = -33139.96 J

E = Qrxn/mass = -33139.96 / 3.457

E = -9586.33 J/g

b)

find moles of H2

Pgas = PTotal - Pvapor = 731-31.8 = 699.2

PV = nRT

n = PV/(RT) = 699.2*0.785 / (62.4*303)

n = 0.0290 mol of Hydrogen gas

c)

M + 3HCl = MCl3 + 3/2H2

1 mol of M = 3/2 mol of H2

x mol of M = 0.0290 mol of H2

x = 2/3*0.0290 = 0.01933 mol of M

MM = mass / mol = 3.457/0.01933 = 178.841 g/mol

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