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Homework Help: Is this subtraction correct

  1. Jul 30, 2009 #1
    a gas contains 2 kinds of atoms 4x105atoms of type A and 8x105 atoms of type B. it is known that the mass of type A atoms is 5x10-26kg and the mass of the gas is 2.8x10-20kg. what is the mass of type B atom?

    2.8x10-20kg - 5x10-26kg = 2.8x10-20kg - 0.00005x10-20 = 2.799995x10-20kg

    is this correct
  2. jcsd
  3. Jul 30, 2009 #2


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    Not really. Think about it. You know the mass of EACH type A atom. You also know how many of them there are. Therefore, you can figure out the total mass due to type A by carrying out a certain arithmetic operation (and, no, it is not subtraction).

    Once you have the total mass of type A, THEN you can subtract it from the total mass of gas, which will give you the total mass due to type B.

    If you know the total mass of the type B gas, and you know how many atoms there are, then the mass PER atom can be calculated using yet another arithmetic operation (that is not subtraction either)
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