# Chemistry A-level Abundance question [Help]

Question: A sample of rubidium contains 85Rb and 87Rb only. The isotope 85Rb has an abundance 2.5 times greater than that of 87Rb
Calculate the relative atomic mass of Rubidium in this sample. Give your answer to one decimal place.
I've looked at the mark scheme and the answer is 85.6, however I have no idea how to get this answer. Can somebody help?
Abundance of 85Rb : 87 Rb = 2.5:1 = 5:2
So, relative atomic mass of Rb
= 85 x 5/7 + 87 x 2/7
= 85.571 a.m.u.
Original post by Äries
Abundance of 85Rb : 87 Rb = 2.5:1 = 5:2
So, relative atomic mass of Rb
= 85 x 5/7 + 87 x 2/7
= 85.571 a.m.u.

Thank you so much! That really helps!
I have one question, it may sound obvious, but why do you not divide the final value by 100?
(edited 11 months ago)
Original post by 06zainab
Thank you so much! That really helps!
I have one question, it may sound obvious, but why do you not divide the final value by 100?

It is because the figure is not a percentage, but number of folds.
Original post by 06zainab
Thank you so much! That really helps!
I have one question, it may sound obvious, but why do you not divide the final value by 100?

The value that its been multiplied by was already divided by 100 so there was no need to do it at the end, as they just used a decimal instead. So instead of doing the ratio as the percentage out of 100 each isotope is they just used the fractions. 5/7 = 0.71528 for example instead of the 71.528% you would use if it gave you it in the question. Eliminating the step of dividing it by 100.
Original post by Maths_Duck
The value that its been multiplied by was already divided by 100 so there was no need to do it at the end, as they just used a decimal instead. So instead of doing the ratio as the percentage out of 100 each isotope is they just used the fractions. 5/7 = 0.71528 for example instead of the 71.528% you would use if it gave you it in the question. Eliminating the step of dividing it by 100.

oh thank you so much! great explanation.
I understand now.