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Homework 63

  • Page ID
    29246
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    Assigned Questions: 10.34,5.75

    Question 10.34:

    Observe the following exchanges of energy. Identify whether each exchange is an example of heat or work. For each system, determine whether ΔE is positive or negative.

    a) A cake is baked in an oven (The cake is the system)

    b) Wood is burned for a fire (The wood is the system)

    c) A boy kicks a chair, it is displaced in the direction of force applied. (The chair is the system)


    Solution

    a) The system is an example of heat, and because the cake gains heat, ΔE is positive.

    b) The system is an example of heat, and because wood gives off heat, ΔE is negative.

    c) The system is an example of work, and because work is done on the chair, ΔE is positive.


    Question 5.75

    Calculate the number of moles in each of the following examples.

    a) 402.5 mg of NO2

    b) 2.7 kg of H2O

    c) 323 g of CBr4

    d) 2.9 kg of CaO


    Solution

    a)

    Explanation:

    First, calculate the molar mass of NO2.

    14.007g/mol N + 2 (15.999 g/mol O) = 46.0055 g/mol

    Now, convert the sample from mg to g.

    402.5 mg NO2 1 g 0.4025 g NO2
    1000 mg

    Finally, use the molar mass to determine how many moles your sample size contains.

    .4025 g NO2 1 mol 0.0087 moles of NO2
    46.0055 g

    This gives us the final answer, 0.0087 moles of NO2.

    From this, we can deduce that:

    Moles of substance = Mass of substance (g) / Molar mass of substance

    b)

    150 moles of H2O

    c)

    0.974 moles of CBr4

    d)

    52 moles of CaO


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