Common brass is a copper and zinc alloy containing 37.0% zinc by mass and having a density of 8.48 g/cm^3?

A fitting composed of common brass has a total volume of 114.5 cm^3. How many atoms of copper does the fitting contain? How many atoms of zinc does the fitting contain?

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  • 7 years ago
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    If the alloy was 37% Zinc, then the Copper content has to be 100 - 37 = 63%. The mass of the object is the product of the volume and density. (8.48 gm/cm^3)*(114.5 cm^3) = 971 grams. The mass of copper is therefore (0.63)*(971 g) = 612 grams and the zinc is the remainder, 971 - 612 = 359 grams of zinc.

    The next part of the problem is what I consider badly taught chemistry, but I'll return to that later. For the moment, one needs to understand what a mole is. It is just a number chemists use to express amount. This many atoms of any element in the periodic table equals its its atomic weight. The answer to the problem involves expressing the given amounts of copper and zinc in terms of moles.

    612 grams / (63.5 grams/mole) = 9.63 moles Cu (copper)

    359 grams / (65.4 grams/mole) = 5.49 moles of Zn (zinc)

    To find the number of atoms, just multiply by the number of atoms in a mole.

    (9.63 moles Cu)*(6.02 x 10^23 atoms/mole) = 5.8 x 10^24 atoms of Cu

    (5.49 moles Cu)*(6.02 x 10^23 atoms/mole) = 3.3 x 10^24 atoms of Zn

    Returning to poorly taught chemistry, it is rather silly to have students count atoms. Chemists never actually use the value of a mole. This is because we express amounts in terms of grams, never atoms. Treating a mole as though it were the number PI just makes chemistry all the harder to understand.

    Source(s): I'm a (working) chemist!
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