Question

if the relative mass of He is taken as 1 unit ,what is that of magnesium ​

Answer 1

Explanation:

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

50

%

abundance.

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

=

∑

i

(

isotope

i

×

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 means that the relative atomic mass of magnesium will be

A

r

=

24 u

×

50

100

+

25 u

⋅

50

100

=

1

2

×

49 u

=

24.5 u