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    My syllabus lists some definitions we need to know but they are incredibly vague and the definitions are not given. They are:

    • amount of substance
    • parts per million (ppm), eg gases in the atmosphere, exhausts, water pollution

    Would anyone be kind enough to explain how Edexcel wish us to define these two things?
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    (Original post by Big-Daddy)
    My syllabus lists some definitions we need to know but they are incredibly vague and the definitions are not given. They are:

    • amount of substance
    • parts per million (ppm), eg gases in the atmosphere, exhausts, water pollution

    Would anyone be kind enough to explain how Edexcel wish us to define these two things?
    your textbook does not provide any details? your school chemistry notes do not provide these details?

    Note: syllabus is meant to be a condensed guide, not the "bible" to all your exam questions!
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    (Original post by shengoc)
    your textbook does not provide any details? your school chemistry notes do not provide these details?

    Note: syllabus is meant to be a condensed guide, not the "bible" to all your exam questions!
    No ... the first I had heard of these two definitions being required was when it said so in the syllabus.
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    (Original post by Big-Daddy)
    No ... the first I had heard of these two definitions being required was when it said so in the syllabus.
    Which section of the syllabus are you looking at?
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    (Original post by charco)
    Which section of the syllabus are you looking at?
    This one is for my AS, so we're looking at the Edexcel Chemistry GCE specification, Unit 1, 1.3 Part C: "demonstrate an understanding of the terms relative atomic mass, amount of substance, molar mass and parts per million
    (ppm), eg gases in the atmosphere, exhausts, water pollution"

    I'm fine with relative atomic mass and molar mass. The other two are too vague for me to pin down with the Internet and I don't want to just come up with random definitions myself for a big exam like this.
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    (Original post by Big-Daddy)
    This one is for my AS, so we're looking at the Edexcel Chemistry GCE specification, Unit 1, 1.3 Part C: "demonstrate an understanding of the terms relative atomic mass, amount of substance, molar mass and parts per million
    (ppm), eg gases in the atmosphere, exhausts, water pollution"

    I'm fine with relative atomic mass and molar mass. The other two are too vague for me to pin down with the Internet and I don't want to just come up with random definitions myself for a big exam like this.
    It doesn't ask you for any definitions ...
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    (Original post by charco)
    It doesn't ask you for any definitions ...
    OK well what am I to understand by the terms "amount of substance" and "parts per million, eg gases in the atmosphere, exhausts, water pollution"? PPM can generally refer to moles or mass, which is unspecified here ... as for "amount of substance", I don't have a clue - is this moles, mass, volume, what?
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    (Original post by Big-Daddy)
    OK well what am I to understand by the terms "amount of substance" and "parts per million, eg gases in the atmosphere, exhausts, water pollution"? PPM can generally refer to moles or mass, which is unspecified here ... as for "amount of substance", I don't have a clue - is this moles, mass, volume, what?
    You are expected to understand the terms, not define them. Your teacher should give you guidance as regards the understanding... but the term "amount of substance" refers to the number of moles.

    ppm may be described in terms of grams, milligrams, micrograms or even moles etc. As it's a relative measurement it really doesn't matter...
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    (Original post by charco)
    You are expected to understand the terms, not define them. Your teacher should give you guidance as regards the understanding... but the term "amount of substance" refers to the number of moles.

    ppm may be described in terms of grams, milligrams, micrograms or even moles etc. As it's a relative measurement it really doesn't matter...
    Thank you.

    I've got an organic question from a STEP paper which I was wondering if you could help with. It says, describe, using examples from the free radical substitution of Br2 onto methane, the meaning of "initiation", "propagation" and "termination" - for 3 marks each. I can write the equations but I think it's also looking for a qualitative description so I'm not really sure what to write.
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    (Original post by Big-Daddy)
    Thank you.

    I've got an organic question from a STEP paper which I was wondering if you could help with. It says, describe, using examples from the free radical substitution of Br2 onto methane, the meaning of "initiation", "propagation" and "termination" - for 3 marks each. I can write the equations but I think it's also looking for a qualitative description so I'm not really sure what to write.
    initiation = starting the free radical mechanism by making free radicals. In the halogenation of alkanes this involves romation of two halogen free radicals by irradiation of halogen molecules with UV light (homolytic fission)

    propagation = continuing the chain reaction. One free radical collides with a molecule to make a new molecule and another free radical.

    termination = finishing. Two free radicals collide and become a molecule. This prevents further chain reaction.

    Typically each free radical formed causes (of the order of) about 100 propagation steps before a termination.

    You can find the free radical mechanism drawn out here.
 
 
 
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