A calorimeter contains 35.0 mL of water at 15.0 C . When 1.70 g of X (a substanc

Question

A calorimeter contains 35.0 mL of water at 15.0 C . When 1.70 g of X (a substance with a molar mass of 76.0 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.

Explanation / Answer

dH = -Q/n

Q = m*Cp*dT

m = mass of water + mass os salt = 35+1.7 = 36.7

dT = 25-15 = 10

Cp = 4.18 (of water, apporx)

then

Q= 36.7*4.18 *10 =1534.06 J

then

n = mass/MW = 1.7/76 = 0.02236842105

n = 0.02236842105 mol

dH = -Q/n = - 1534.06 /0.02236842105

dH = - 68581.5058 J/mol

d H= - 68. 58 kJ/mol

3 sig fig

dH = -6.86*10^3 kJ/mol

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