A calorimeter contains 20.0 mL of water at 13.5 C . When 1.30 g of X (a substanc

Question

A calorimeter contains 20.0 mL of water at 13.5 C . When 1.30 g of X (a substance with a molar mass of 52.0 g/mol ) is added, it dissolves via the reaction X(s)+H2O(l)X(aq) and the temperature of the solution increases to 25.5 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

Explanation / Answer

X(s)+H2O(l)X(aq)

The mass of water = density * volume

= 1.00 g/ mL*20.0 mL

= 20.0 g

Total mass of solution = 20.0+1.30 )g

= 21.30 g

Now calculate the heat ;q = mc*dT

= 21.30 g * 4.18 J/(gC * [25.5-13.5] C

= 1068.408 J

Now calculate the number of moles = 1.30 g/ 52 g/ mol

= 0.025 moles

To calculate the enthalpy change, H, for this reaction per mole of X divide total heat by the number of moles:

= 1068.408 J /0.025 moles

= 42736.32 J/ mol

= 42.74 KJ/ mol

thus  the enthalpy change, H, for this reaction is -42.74 KJ/ mol becasue this is an exothermic reaction

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