Question

1. estimate the actual atomic mass of ²⁴mg.

isotope	mass (amu)	relative abundance (%)
neon - 21	20.994
neon - 22		9.22
avg. atomic mass =		total %:

Explanation:

Step1: Convert percentages to decimals

$90.51\% = 0.9051$, and assume the relative - abundance of Neon - 21 is $x$, then $x=1-(0.9051 + 0.0922)=0.0027$.

Step2: Calculate the average atomic mass

The formula for average atomic mass $A=\sum_{i}m_i\times p_i$, where $m_i$ is the mass of the isotope and $p_i$ is its relative abundance.
$A=(19.992\times0.9051)+(20.994\times0.0027)+(21.991\times0.0922)$
$A = 19.992\times0.9051+20.994\times0.0027 + 21.991\times0.0922$
$A=18.0957692+0.0566838+2.0275702$
$A = 20.1799232\approx20.18$ amu.

Answer:

$20.18$ amu