[jennielynn_1980]

I am trying to find the number of atoms in the following

a) 52.5 g calcium metal
b) 1.5 mol carbon
c) 1.5 mol carbon dioxide

So I have to find the molar mass for each before proceding right?  In which case the molar mass for
a) is 40.078 g/mol
b) 12.0107 g/mol
c) How do I find the molar mass for this?  Is it the molar mass for carbon plus the molar mass for oxygen multiplied by 2?

Thanks

[mike]

Yes

[syko sykes]

then use avagodro's (hope i spelled that right) number to convert moles to atoms

in other words, multiply number of moles by 6.02x10²³

[mike]

avagodro's (hope i spelled that right)

Nope

[syko sykes]

avagodro's (hope i spelled that right)

Nope

Avogadro's... that's a little better

and if you want to be more precise, Avogadro's number = 6.0221415 × 10²³

[jennielynn_1980]

So for

a) the answer would be 0.7394
b) 1.9999
c) 1.4999

Is this right?  Or am I missing a step?

[rctrackstar2007]

aren't the units atoms/mol??

if that's correct then your answers would be wrong

[tennis freak]

for the first i got 7.89E23
second and third it should be 9.033E23
because you take moles*avogadro's number to get atoms and the second and third are already in moles so they need no change.

[syko sykes]

for the first i got 7.89E23
second and third it should be 9.033E23
because you take moles*avogadro's number to get atoms and the second and third are already in moles so they need no change.

b and c wouldn't be the same because one molecule contains a different number of atoms. b is probably double your answer because pure carbon typically bonds with itself to form C₂. C is probably triple your answer because there are 3 atoms (1 carbon and 2 oxygen) in one molecule of carbon dioxide. I say probably on these because i didn't actually calculate, i'm just assuming you took 1.5*6.02*10^23.

[rctrackstar2007]

A i got, 4.7E23

40g/mol / 52.5g x 6.2E23atoms/mol leaves 4.7E23atoms

[Will]

tennis freak is right for A & B and syko sykes got C right.
A is: (52.5 / 40.078) x (6.0221415 × 10²³) which is 7.889 x 10²³ atoms.
For B and C you don't need to worry about M_r of carbon etc. because it already tells you the moles. For B it fair to assume that they mean carbon atoms because there are so many allotropes of carbon (graphite (and diamond to some extent) being the most common), you don't know which one they are talking about. It is also possible to make carbon atoms but it is extremely difficult and dangerous.
Hence,
B is: 1.5 x (6.0221415 × 10²³) which is 9.033 x 10²³ atoms.
For C you multiply this by 3 because there are 3 atoms in a CO₂ molecule.
C is: 3 x 1.5 x (6.0221415 × 10²³) which is 2.71 x 10²⁴ atoms.

[jennielynn_1980]

Thank you all!  I was confused with the wording of it all.  I have never taken chemistry before and I am enrolled in advanced 12th grade chem  so my head is spinning a bit as it is also a home study course.

[tennis freak]

believe me this website would have helped me so much more if i had found it sooner, so just keep asking questions on here and you should be fine

[jennielynn_1980]

I already love this site!  Everyone here is very helpful and friendly

One last question about this:

if one mole  = 6.02 x 10²³ of any substance, then why do I multiply 1.5 moles of CO₂ by three?  Should the answer not be the same as the second seeing as the moles are the same?  I can see that there are two oxygens and one carbon but if it is still 1.5 moles then shouldn't the number of particles be the same?

[Will]

if one mole  = 6.02 x 10²³ of any substance, then why do I multiply 1.5 moles of CO₂ by three?  Should the answer not be the same as the second seeing as the moles are the same?  I can see that there are two oxygens and one carbon but if it is still 1.5 moles then shouldn't the number of particles be the same?

You understand the chemistry perfectly- there are the same number of particles in B as C, but the question is asking about the number of atoms (I think!), and there are 3 atoms in one CO₂ particle, so you multiply it by three. If the question asked for the number of particles then B would be the same as C.