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5 September, 22:26

A student is preparing to perform a series of calorimetry experiments. She first wishes to determine the calorimeter constant (Ccal) for her coffee cup calorimeter. She pours a 50.0 mL sample of water at 345 K into the calorimeter containing a 50.0 mL sample of water at 298 K. She carefully records the final temperature of the water as 317 K. What is the value of Ccal for the calorimeter? A student is preparing to perform a series of calorimetry experiments. She first wishes to determine the calorimeter constant (Ccal) for her coffee cup calorimeter. She pours a 50.0 mL sample of water at 345 K into the calorimeter containing a 50.0 mL sample of water at 298 K. She carefully records the final temperature of the water as 317 K. What is the value of Ccal for the calorimeter? 99 J/K 21 J/K 76 J/K 28 J/K 19 J/K

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  1. 6 September, 00:43
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    First choice: 99 J/K

    Explanation:

    1) First law of thermodynamic (energy balance)

    Heat released by the the hot water (345K) = Heat absorbedby the cold water (298 K) + Heat absorbed by the calorimeter

    2) Energy change of each substance:

    General formula:

    Heat released or absorbed = mass * Specific heat * change in temperature

    density of water: you may take 0.997 g / ml as an average density for the water.

    mass of water: mass = density * volume = 50.0 ml * 0.997 g/ml = 49.9 g

    Specif heat of water: 1 cal / g°C

    Heat released by the hot water:

    Heat₁ = 49.9 g * 1 cal / g°C * (345 K - 317 K) = 49.9 g * 1 cal / g°C * (28K)

    Heat absorbed by the cold water:

    Heat₂ = 49.9 g * 1 cal / g°C * (317 K - 298 K) = 49.9 g * 1 cal / g°C * (19K)

    Heat absorbed by the calorimeter

    Heat₃ = Ccal * (317 K - 298 K) = Ccal * (19K)

    4) Balance

    Heat₁ = Heat₂ + Heat₃

    49.9 g * 1 cal / g°C * (28 K) = 49.9 g * 1 cal / g°C * (19 K) + Ccal * (19 K)

    Solve for Ccal

    Ccal = [49.9 g * 1 cal / g°C * (28 K) - 49.9 g * 1 cal / g°C * (19 K) ] / 19K

    Ccal = 23.6 cal / K

    Convert to cal / K to Joule / K

    1 cal = 4.18 Joule

    23.6 cal / K * 4.18 J / cal = 98.6 J/K

    Which rounded to 2 signficant figures leads to 99 J/k, which is the first choice.
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