Help chemistry mcq

Help with this mcq I don’t get how the answer is D.
I did

(11 * x) + (10 *x) / 100 =10.8
And I don’t get what the answer is because there are 2 of the percentages for both B missing

(edited 1 year ago)

Is the answer not C?

Original post by ghostyblop

Is the answer not C?

Yeah sorry it is c, I had the wrong ms 😭.
But I still don’t understand the question can u plz help me out

Reply 3

Ðeggs

A sample of Boron has Boron 10 and Boron 11. The relative atomic mass of the sample is 10.8, therefore you should recognise by simple mathematical reasoning that this is likely to be 80% of Boron 11 and 20% of Boron 10. (0.8 x 11 + 0.2 x 10 = 10.8). Therefore the percentage of Boron 11 in the sample is 80%, which is option C.

(edited 1 year ago)

Reply 4

Alevelhelp.1

Original post by Ðeggs

Ohhh that’s such a weird q thanks

(edited 1 year ago)

Reply 5

ghostyblop

Original post by Alevelhelp.1

Yeah sorry it is c, I had the wrong ms 😭.
But I still don’t understand the question can u plz help me out

No worries :smile:

sure!
I think what you've done wrong is assume that the two isotopes are in the same ratio.

If you do: avg atomic weight= (percentage abundance isotope 1)*mass of isotope 1+ (percentage abundance isotope 2)*mass of isotope 2

Then plugging in our numbers we get
10.8=(PA isotope 1)*10 + (PA isotope 2)*11

We can define the PA of each isotope algebraically

If we let the percentage abundance of isotope 1: x
Then the percentage abundance of isotope 2 must be: 1-x

Therefore: 10.8= 10x + (1-x)*11
10.8=10x+11-11x
-0.2=-x

So x= 20% x is the percentage abundance for isotope 1 i.e. B10
Thus the percentage abundance for B11 will be 80%

Which is basically what the above comment did, sometimes having a method is easier than spotting relationships between numbers even if it does take longer- at least that's what i find lol!

Hope that helped, let me know if this makes sense :smile: