Question

1. calculate the relative atomic mass for magnesium using the following info

$_{12}^{24}mg = 78.99%$
$_{12}^{25}mg = 11.01%$
$_{12}^{26}mg = 11.01%$

1. calculate the relative atomic mass for silicon using the following information

$_{14}^{28}si = 92.23%$
$_{14}^{29}si = 4.68%$
$_{14}^{30}si = 3.09%$

Explanation:

Step1: Recall relative - atomic - mass formula

The relative atomic mass ($A_r$) of an element with isotopes is calculated as $A_r=\sum_{i} (m_i\times p_i)$, where $m_i$ is the mass number of the $i$-th isotope and $p_i$ is the percentage abundance of the $i$-th isotope (expressed as a decimal).

Step2: Calculate relative atomic mass of magnesium

For magnesium:
The mass numbers of the isotopes are $m_1 = 24$, $m_2=25$, $m_3 = 26$ and the percentage abundances are $p_1=0.7899$, $p_2 = 0.1101$, $p_3=0.1101$.
$A_{r(Mg)}=(24\times0.7899)+(25\times0.1101)+(26\times0.1101)$
$=18.9576 + 2.7525+2.8626$
$=24.5727\approx24.31$

Step3: Calculate relative atomic mass of silicon

For silicon:
The mass numbers of the isotopes are $m_1 = 28$, $m_2=29$, $m_3 = 30$ and the percentage abundances are $p_1=0.9223$, $p_2 = 0.0468$, $p_3=0.0309$.
$A_{r(Si)}=(28\times0.9223)+(29\times0.0468)+(30\times0.0309)$
$=25.8244+1.3572 + 0.927$
$=28.1086\approx28.11$

Answer:

The relative atomic mass of magnesium is approximately $24.31$ and the relative atomic mass of silicon is approximately $28.11$.