Question

there are two stable isotopes of gallium, ^69ga and ^71ga. 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
calculate and select the answer with the 2 correct mass contributions
isotope % abundance relative mass mass contribution
ga - 69 60.1089 68.925580 amu 4143.0408 amu
ga - 71 39.8929 70.924701 amu 2829.3920 amu
isotope abundance relative mass mass contribution
ga - 69 0.601089 68.925580 amu 41.430408 amu
ga - 71 0.398929 70.924701 amu 28.293920 amu

Explanation:

Step1: Convert percentage to decimal

To calculate the mass - contribution, we need to convert the percentage abundance to a decimal. For Ga - 69, the decimal abundance $a_{69}=60.1089\div100 = 0.601089$. For Ga - 71, the decimal abundance $a_{71}=39.8929\div100=0.398929$.

Step2: Calculate mass - contribution of Ga - 69

The mass - contribution of an isotope is the product of its decimal abundance and its relative mass. For Ga - 69, with relative mass $m_{69}=68.925580$ amu, the mass - contribution $MC_{69}=a_{69}\times m_{69}=0.601089\times68.925580\approx41.430408$ amu.

Step3: Calculate mass - contribution of Ga - 71

For Ga - 71, with relative mass $m_{71}=70.924701$ amu, the mass - contribution $MC_{71}=a_{71}\times m_{71}=0.398929\times70.924701\approx28.293920$ amu.

Answer:

isotope abundance relative mass mass contribution
Ga - 69 0.601089 68.925580 amu 41.430408 amu
Ga - 71 0.398929 70.924701 amu 28.293920 amu