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25 May, 07:33

The temperature of 500.0 g of water contained in a calorimeter increased from 20.0 °C to 50.0 °C when 5.00 g of fuel was burned to completion. What is the ΔH of this combustion reaction? (The specific heat of water is 4.184 J/g°C and the molecular weight of the fuel is 40.0 g/mol.)

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  1. 25 May, 07:58
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    The ΔH of this combustion reaction is - 502 kJ/mol

    Explanation:

    Step 1: Data given

    Mass of water = 500.0 grams

    Mass of fuel = 5.00 grams

    Initial temperature = 20.0 °C

    Final temperature = 50.0°C

    The specific heat of water is 4.184 J/g°C

    Molecular weight of the fuel is 40.0 g/mol

    Step 2: Calculate the heat transfer

    q = m*c * ΔT

    ⇒ with m = the mass of water = 500 grams

    ⇒ with c = the specific heat of water = 4.184 J/g°C

    ⇒ with ΔT = the change in temperature = T2 - T1 = 50.0°C - 20.0°C = 30.0°C

    q = 500 * 4.184 * 30

    q = 62760 J

    Step 3: Calculate moles

    moles = mass / molar mass

    Moles = 5 grams / 40 g/mol

    moles = 0.125 moles

    Step 4: Calculate ΔH of this combustion reaction

    ΔH = 62760 / 0.125 mol

    ΔH = 502080 J/mol = 502 kJ / mol (negative because it's an exothermic reaction)

    ΔH = - 502 kJ/mol
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