[shout1]

I just had this question on a quiz and I'm not sure what I did wrong. ( I got it wrong)
Question: Calculate the number of hydrogen atoms in 0.125 g of citric acid H₃C₆H₅O₇

Heres what I did..
I found the percent composition of hydrogen to be 4.17%, then I found the total moles of hydrogen in citric acid and multiplied it by 0.0417 (this is where I think i went wrong), then multiplied that number by avagadros number to get 1.63 X 10¹⁹ particles.

Anyone care to correct me and provide right answer?

Thanks.

[Jorriss]

That is where you went wrong, multiplying the number of moles by percent composition won't give anything helpful as it is not eliminating moles or mass.

Here's a more useful way to do this problem.

Find the moles of citric acid, for every mole of citric acid there are three moles of hydrogen and from there...

[Borek]

No need for percent composition.

How many moles of citric acid? How many H atoms per one molecule of citric acid? Hint: it is not three...

[shout1]

Okay I tried it this way:

n=m/mm
n=0.125/192
= ~0.000651 moles of citric acid

for every 1 mole of citric acid there are 8 moles of hydrogen
for 0.000651 moles of citric acid there are x moles of hydrogen.
1:8
0.000651:x
x= 0.0052 moles of hydrogen

0.0052 X 6.02 X 10²³
 = 3.135 X 10²¹ particles.

Is that right?

[Jorriss]

How many moles of citric acid? How many H atoms per one molecule of citric acid? Hint: it is not three...

lol... I suck, I did not look past those first three hydrogens.

[Borek]

3.135 X 10²¹ particles.

Is that right?

Yes.