How would you find the relative atomic mass of magnesium if there is 1 atom of Mg-24 to every atom of Mg-25?

Question

Henry Antrim · Accepted Answer

#"24.5 u"#

The concept here is that, although presented in an evasive manner, the question gave you all the details you require to determine the relative atomic mass of magnesium.

The first thing you'll notice is that the two isotopes you're given are magnesium-24 and magnesium-25.

The question also provides you with their abundances, which is the second thing to notice.

one magnesium-24 atom for each magnesium-25 atom

tells you that both isotopes contribute equally to the relative mass of magnesium, or, in other words, both isotopes have a #50%# abundance.

The atomic masses of an element's isotopes and their corresponding abundances can now be used to determine the element's relative atomic mass.

#A_"r" = sum_i("isotope"_i xx "abundance"_i)#

In your case, magnesium-24 will have an atomic mass of #"24 u"# because it contains 12 protons and 12 neutrons. Here #u# represents the unified atomic mass unit and is equal to the mass of one nucleon (proton or neutron).

Likewise, magnesium-25 will have an atomic mass of #"25 u"# because it contains 12 protons aqnd 13 neutrons.

This indicates that magnesium's relative atomic mass will be

#A_"r" = "24 u" xx 50/100 + "25 u" * 50/100 = 1/2 xx "49 u" = color(green)("24.5 u")#

Serenity Allen · Answer

undefined Multiply the atomic mass of both Mg-25 and Mg-24 by their respective abundances (1) and (1), then add the results.