Question

silicon is the second most abundant element in the earths crust. it is used in the electronics industry to produce concrete and cement. the graph below shows the relative abundance of the naturally occurring silicon isotopes. based upon the graph would the estimated average atomic mass of silicon? do not calculate
26.2 amu
28.1 amu
29.8 amu
30.1 amu
silicone isotope percent abundance
si - 30 3.1%
si - 29 4.7%
si - 28 92.2%

Explanation:

Step1: Identify isotope abundances

Si - 28 has 92.2% abundance, Si - 29 has 4.7% abundance, Si - 30 has 3.1% abundance. Assume Si - 28 mass is 28 amu, Si - 29 mass is 29 amu, Si - 30 mass is 30 amu for estimation.

Step2: Use average - atomic - mass formula

The formula for average atomic mass $\bar{A}=\sum_{i = 1}^{n}A_i\times p_i$, where $A_i$ is the mass of isotope $i$ and $p_i$ is the percent - abundance of isotope $i$ (in decimal form).
$p_{28}=0.922$, $p_{29}=0.047$, $p_{30}=0.031$.
$\bar{A}=(28\times0.922)+(29\times0.047)+(30\times0.031)$
$=25.816 + 1.363+0.93$
$=28.109\approx28.1$ amu

Answer:

28.1 amu