1) A calorimeter contains 35.0 mL of water at 12.5 C . When 1.20 g of X (a subst

Question

1) A calorimeter contains 35.0 mL of water at 12.5 C . When 1.20 g of X (a substance with a molar mass of 43.0 g/mol ) is added, it dissolves via the reaction X(s)+H2O(l)X(aq) and the temperature of the solution increases to 25.0 C . Calculate the enthalpy change, H, for this reaction per mole of X. Assume that the specific heat of the resulting solution is equal to that of water [4.18 J/(gC)], that density of water is 1.00 g/mL, and that no heat is lost to the calorimeter itself, nor to the surroundings. Express the change in enthalpy in kilojoules per mole to three significant figures.

2) A silver block, initially at 56.2 C, is submerged into 100.0 g of water at 24.0 C, in an insulated container. The final temperature of the mixture upon reaching thermal equilibrium is 27.8 C. What is the mass of the silver block? Express your answer to two significant figures and include the appropriate units.

Explanation / Answer

(1) Given mass of X is 1.20 g

Mass of water is volume x density

                        = 35.0 mL x 1.00 g/mL

                        = 35.0 g

Change in temperature , dt = final - initial = 25.0-12.5 = 12.5 oC

The amount of heat released , q = mcdt

Where m = mass of water = 35.0 g

c= specific heat of water = 4.18 J/(gC)

Plug the values we get q = 35.0x4.18x12.5 = 1828.75 J

                                                              = 1.83 kJ               Since 1 kJ = 1000 J

This is the amout of heat released by 1.20 g of X

1.20 g of X releases 1.83 kJ of heat

43.0 g of X releases M kJ of heat

M = ( 1.83x43.0) / 1.20

    = 65.5 kJ

Therefore the enthalpy change, H = -65.5 kJ/mol( the negative sign indicated that heat is releasing)

(2) The heat released by silver block = heat gained by water

                                        mcdt = m'c'dt'

Where

m = mass of silver = ?

c = specific heat of silver = 0.240 J/goC

dt = change in temperature = initial - final = 56.2 - 27.8 = 28.4 oC

m' = mass of water = 100.0 g

c' = specific heat of water = 4.18J/goC

dt' = change in temperature of water = final - initial = 27.8 - 24.0 = 3.8 oC

Plug the values we get m = ( m'c'dt') / ( cdt)

                                     = 233.0 g

Therefore the mass of silver block is 233.0 g

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