A calorimeter contains 22.0mL of water at 11.0?C . When 2.30g of X (a substance

Question

A calorimeter contains 22.0mL of water at 11.0?C . When 2.30g of X (a substance with a molar mass of 42.0g/mol ) is added, it dissolves via the reaction

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

and the temperature of the solution increases to 30.0?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/(g??C)], 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

q=mct
where q is the change in energy(delta h), t is the change in temperature, c is the specific heat of water, and m is the total mass of the solution. the density of water is ~1g /ml.

q= (36.2 g) x (4.18 J/(g*C))(16.5 C)
= 2497 J

you then find the number of moles of X. the formula is

mass of x/molar mass of x = number of moles

2.3 g / (42 g/mol) = .05476 moles

then you divide the total energy by the number of moles to find the enthalpy change per mole

2497 J / .05476 moles = 4.56 x 10^4 J /mole= 45.6 KJ/mole

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