Question

there are two stable isotopes of gallium, $^{69}ga$ and $^{71}ga$. calculate the average atomic mass from the following data from the mass spectrometer.
isotope % abundance relative mass mass contribution
ga - 69 60.1089 68.925580 amu
ga - 71 39.8929 70.924701 amu
do you use the % abundance or the abundance?
abundance
% abundance
neither

Explanation:

Step1: Convert % abundance to abundance

To calculate the average atomic mass, we use the abundance (not % abundance). We divide the % abundance by 100. For Ga - 69, abundance $a_1=\frac{60.1089}{100}=0.601089$, for Ga - 71, abundance $a_2=\frac{39.8929}{100}=0.398929$.

Step2: Calculate mass - contributions

The mass - contribution of an isotope is the product of its abundance and its relative mass. For Ga - 69, mass - contribution $m_1=a_1\times68.925580 = 0.601089\times68.925580\approx41.487$. For Ga - 71, mass - contribution $m_2=a_2\times70.924701=0.398929\times70.924701\approx28.208$.

Step3: Calculate average atomic mass

The average atomic mass $M = m_1 + m_2$. So $M=41.487+28.208 = 69.695$.

Answer:

abundance