What is the percentage of elements in a compound?

[Positronix]

TL;DR Summary

percentage of elements in a compound without involving molarity?

Thanks in advance for explaining this to me.

I have a very small sample of thorium dioxide in a sealed ampoule (reasonably pure) as an example of thorium for a display.

Every time I wonder what % of the compound is thorium people start talking moles and grams and envisioning it in a solution...

That's great to be able to do all that, and I'll re-acquaint myself with it (I used to do the equations back in college and forgot)
but right off the top of one's head:

Why can't I just say:
the formula for the compound is Th02
1 thorium and 2 oxygens
On the periodic table:
thorium has a mass of 90
oxygen has a mass of 8 (and there's two, so we'll say 8*2=16)

90 / 16 = 5.625
therefore there is 5.625 times as much Th as there is O in ThO2
for %, 100 / 5.625 = 17.778 % Oxygen
100 - 17.778= 82.222% Thorium

and if this is correct, why do I even have to get into molarity for dry weight?
If I have 1 gram of ThO2 why can't I just use the above to say it contains .822 g of Th?

Thanks again.

 

[Charles Link]

You need to use atomic weight, (and not atomic number)=thorium is 232. Two oxygens gives 32. The percentage thorium is ## (232/(232+32))100. (This step you did incorrectly in the above). e.g. if there is 4x as much thorium as oxygen, it is then 4/5 thorium and 1/5 oxygen.

 

[Positronix]

Thanks, I learned 2 important things from your refresher and corrections.
3 things, really -- my silliest error was thinking that a periodic table that was only showing one number (atomic number as it turns out) was showing atomic weight... I hate those! I should have known 90 was too small for Th.

But I realized from your help:
My primary error I'd have continued to make:
I needed to use the whole 232 + 32 for division, not just the larger atom's mass number.
(thanks)

Incidently, bane of my dyslexic existence:
I also (and this gets into my being away from basic math too long with 2x diagnosed dyslexia)
I keep wanting to do extra math because of how my brain works, which just takes longer and opens an extra door for error...

Using ambiguous #'s for demonstration:
If I want to know 3 is what % of 12 (this is easy in its simplicity, but I miss this "shortcut" with larger #'s)
I should not bother to say
12/3 = 4
then
100/4=25
then
25/100=.25

when I can just
3/12=.25 !
for some reason I never intuitively realize I can do this.
My consciousness can fully "see" a 12 is made up of four 3's and 100/4 = 25
but I simply have to memorize that you can always reverse the division to get the % directly.
Conceptually I guess I know division is the counterpart of multiplication...

I'm just trying to beat things into my conscious, intuitive, automatic awareness because, if I wrap my brain around something 360 degrees, it works much better (years from now) than merely memorizing "that's just the way it is". Mere memorization has gotten other people very fasr but I cannot reky on my recall thusly.

If 12/3=4 (and 4 happens to be 25% of 100 because 1/4=.25) then 3/12 will always be .25 !

[100/(12/3)]/100 = 3/12 !

Haha... thanks for helping me get back into the groove.

You helped me 2 if not 3 ways in one, fell swoop.