Use the mass spectrum of rubidium to determine the atomic mass of rubidium.

Question

Mass spectrum of rubidium showing intensity percentages at m/z values 203, 204, and 205.

Accepted Answer

Welcome back everyone in this example, we need to determine the atomic mass of thallium from the mass spectrum of thallium shown below. So we want to recall that to calculate atomic mass, we would take the sum of the contribution of each of its isotopes. So we would find that by taking the average peak intensity of each isotope, multiplied by its atomic mass. And we would see that we can calculate average peak intensity by taking our given peak intensity for our isotope and dividing that by our total peak intensity. So recall that we calculate total peak intensity from our graph, where we would see for our first isotope of thallium, which would be thallium 203, we have a peak intensity of 41.89 and we'll round this to a value of about 42%. So we would have 42 added to our second isotope of thallium recorded which is thallium 205 corresponding to a peak intensity of 100%. So we would say 42 plus 100. And this gives us a total peak intensity of 142. And so our next step is to calculate our contribution of each isotope and its atomic mass. So for thallium TL203 We would find its atomic mass by taking its peak intensity in the numerator, which were rounded to about 42 divided by the total peak intensity being 1.42. And we're going to multiply by the Mass of this isotope given in the name of the Isotope being 203. And so in doing so, we would get its contribution or its atomic mass of 60.04-2. Moving on to our second isotope of thallium thallium 205 recorded on the graph we see above that it has a peak intensity of 100. So we place that in the numerator we divide by the total peak intensity which above we calculated as 1 42. And then we will multiply by this isotopes mass given in its name being 205. And this will give us a atomic mass for thallium 205 equal to 1 44.3662. So these two values here are the contribution to the atomic mass of our isotopes of thallium. And so we would say that to calculate the atomic mass of thallium, we would take the contribution from each of the above isotopes that we just calculated. So we would take 60.422 added to 1 44.3662. And this some gives us our atomic mass equal to 204.4 am use which we can round to about 204 am us. And so 204 would be our final answer as our atomic mass to complete this example for thallium. So I hope that everything I explained was clear. If you have any questions, please leave them down below and I will see everyone in the next practice video.

Use the mass spectrum of rubidium to determine the atomic mass of rubidium.

Verified Solution

Key Concepts

Mass Spectrum

Atomic Mass

Isotopes