MAGNESIUM has 3 ISOTOPES of differing mass and in different proportions shown below . Which option shows how we calculate its relative atomic mass (A_(r)) in g/mol {}^24Mg:79% {}^25Mg:10% {}^26Mg:11% ((179+10+11)times (24+24+26))/(100)=75 ((79+24)times (10+25)times (11+26))/(100)=1333.9 ((79times 24)+(10times 25)+(11times 26))/(100)=24.3 ((79times 10times 11)+(24times 25times 26))/(100)=242.9

<br />To find the relative atomic mass (Ar) in g/mol, we should take a weighted average based on the percent abundance of each isotope. <br /><br />The percent abundances need to be converted into decimals (i.e., divide the percents by 100). So, 79% would be 0.79 or 79/100, 10% would be 0.1 or 10/100, and 11% would be 0.11 or 11/100. The isotopes are multiplied by their abundances, added together, and this sum is the average atomic mass (Ar).<br /><br />1. For ^24Mg, multiply its percent abundance 79/100( or 0.79) by its mass (24g/mol). Therefore, the contribution to the average atomic mass from ^24Mg would be 0.79 * 24 = 18.96 g/mol.<br />2. For ^25Mg, multiply its percent abundance 10/100 (or 0.1) by its mass (25g/mol). Its contribution is then 0.1 * 25 = 2.5 g/mol.<br />3. Similarly for ^26Mg, multiply 11/100 (or 0.11) by 26, the mass of ^26Mg, to get a contribution of 0.11 * 26 = 2.86 g/mol.<br />4. Add the contributions together: 18.96 + 2.5 + 2.86 = 24.32 g/mol. <br /><br />Therefore, choice (C), which calculates as follows<br />\[<br />\frac{(79 \times 24)+(10 \times 25)+(11 \times 26)}{100} = 24.3 \ g/mol,<br />\]<br />evinces the correct method to calculate the relative atomic mass, although it rounds to 24.3 rather than the more precise 24.32.