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28 June, 11:12

The reaction nacl (s) → nacl (aq) is performed in a coffee cup calorimeter, using 100 ml of h2o (l) and 5.00g of nacl. if the temperature of solution after mixing decreased by 2.30°c, what is the δhrxn in kj/mol? assume the heat capacities of all solutions are 4.18 j/g°c, and densities of all solutions are 1.00 g/ml. the molar mass of nacl is 58.44 g/mol

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  1. 28 June, 11:24
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    Since the density of water is 1 g / mL, hence there is 100 g of H2O. So total mass is:

    m = 100 g + 5 g = 105 g

    => The heat of reaction can be calculated using the formula:

    δhrxn = m C ΔT

    where m is mass, C is heap capacity and ΔT is change in temperature = negative since there is a decrease

    δhrxn = 105 g * 4.18 J/g°C * (-2.30°C)

    δhrxn = - 1,009.47 J

    => However this is still in units of J, so calculate the number of moles of NaCl.

    moles NaCl = 5 g / (58.44 g / mol)

    moles NaCl = 0.0856 mol

    => So the heat of reaction per mole is:

    δhrxn = - 1,009.47 J / 0.0856 mol

    δhrxn = - 11,798.69 J/mol = - 11.8 kJ/mol
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