Methane has a constant composition of 25% hydrogen and 75% carbon. Using a value of 12 amu for a carbon atom and assuming that methane contains 2 hydrogen atoms and 1 carbon atom, determine the relative mass of a hydrogen atom.
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This methane molecule has 2 hydrogens and 1 carbon. The carbon is 12 amu, and the problem tells us that methane as a whole is 75% carbon. We can use this information to find the mass of the WHOLE methane molecule.
(0.75)(x) = 12 amu
x = 16 amu
Now that we know the whole methane molecule is 16 amu, how much of that is made of hydrogens? 16 amu - 12 amu = 4 amu.
4 amu is all hydrogens. How many hydrogens are there? There are 2. So what is the mass of EACH hydrogen? 4 amu / 2 = 2 amu. There you go!
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