Question

If phosphorus (P) has 4 naturally occurring isotopes, phosphorus-29(32.7.%), phosphorus-30(48.03%), phosphorus-31(18.4%), and phosphorus-33 (0.87%), what is its average r.a.m.?
A. 29.9
B. 123
C. 100
D. 30.75

Answer 1

Answer:

A. 29.9

Explanation:

• An element can have differing numbers of neutrons in its nucleus, but it always has the same number of protons. The versions of an element with different neutrons have different masses and are called isotopes.

• The average atomic mass for an element is calculated by summing the masses of the element’s isotopes, each multiplied by its natural abundance on Earth.

Average atomic mass of P = ∑(Isotope mass)(its abundance)

∴ Average atomic mass of P = (P-29 mass)(its abundance) + (P-30 mass)(its abundance) + (P-31 mass)(its abundance) + (P-33 mass)(its abundance)

Abundance of isotope = % of the isotope / 100.

∴ Average atomic mass of P = (29)(0.327) + (30)(0.4803) + (31)(0.184) + (33)(0.0087) = 29.88 a.m.u ≅ 29.9 a.m.u.

So, the right choice is: A. 29.9