How would you find the relative atomic mass of magnesium if there is 1 atom of Mg-24 to every atom of Mg-25?
The idea here is that the question provided you with all the information you need to find the relative atomic mass of magnesium, but it did so in a tricky way.
The first thing to notice that you're only given two isotopes, magnesium-24 and magnesium-25.
The second thing to notice is that the question also gives you their abundances. The phrase
one atom of magnesium-24 to every one atom of magnesium-25
tells you that both isotopes contribute equally to the relative mass of magnesium, or, in other words, both isotopes have a
Now, the relative atomic mass of an element can be found by using the atomic masses of its isotopes and their respective abundance
#A_"r" = sum_i("isotope"_i xx "abundance"_i)#
In your case, magnesium-24 will have an atomic mass of
Likewise, magnesium-25 will have an atomic mass of
This means that the relative atomic mass of magnesium will be
#A_"r" = "24 u" xx 50/100 + "25 u" * 50/100 = 1/2 xx "49 u" = color(green)("24.5 u")#