moles question (1 Viewer)

[clintmyster]

Q: Calculate the mass of one atom of zinc.

I am first calculating the number of moles in zinc >>> # of atoms = # mols / (6.02 x 10^23)

then i am subbing the number of moles into the equation number of moles = mass of substance over Molar mass

6.02x10^23 = m/M

m = 3.94x10^25 but answer says 1.09x10^-22


I just dont seem to be getting the right answer for this question. I dont understand why the answer is 1.09x10^-22 and not 3.94x10^25.

please help

 

[boris]

No, okay I got the right answer.

On your calculator make sure you have scientific notation.

65.41/6.022E23 = 1.09^-22

Where 65.41 is the atomic weight of zinc and 6.022^23 is Avagadros Number.

You're pressing something wrong on the calculator. Make sure you use the EXP function for your power of 10 instead of the ^ function, as this is often where mistakes are made.
But 1.09x10^-22 is definitely the answer.

Another problem I see is that you're calculating the numbers of moles. You only need to do this when you've been given an amount. If they said 'find the mass of one atom of zinc from a 89.56g sample) thats when you would calculate moles. But if you're just given zinc and no amount, you can safely assume you're working with the atomic weight of one zinc, in which case it's as simple as diving the atomic weight by avagadros number.

 

[lolokay]

Q: Calculate the mass of one atom of zinc.

I am first calculating the number of moles in zinc >>> # of atoms = # mols / (6.02 x 10^23)

then i am subbing the number of moles into the equation number of moles = mass of substance over Molar mass

6.02x10^23 = m/M

m = 3.94x10^25 but answer says 1.09x10^-22


I just dont seem to be getting the right answer for this question. I dont understand why the answer is 1.09x10^-22 and not 3.94x10^25.

please help

you're calculating the mass of an atom; obviously it's going to be a really small number, not a really big one
I think you've gotten the m and M confused - as you've written it, m is the molar mass, M is the atom's mass. You've entered them the wrong way around (ie multiplied molar mass by number of atoms, rather than dividing it)

 

[clintmyster]

i understand it now..my formula was wrong. I got another question.

a crystal of table sugar (sucrose, C12H22O11) weighs 1.4mg. How many molecules are in it? How many atoms in total are present?

i worked out the number of molecules being 2.46 x 10^18 but how do you work out the number of atoms present? Would the Molar Mass be the addition of the RAM and you substitute that in place of the RMM as used in the first part of the question?

 

[minijumbuk]

Here's an image I drew up just then xD

I hope it makes your chemistry calculating-life much more easier.

Oh, the bottom two equations will be taught in year 12.

 

[boris]

Also a really easy way of remembering is to draw a triangle.

Say you want molarity, you put M at the top, put a line underneath it, divde the bottom segment into two and put m for moles and v for volume (respectively from left to right) in the segments.

That way you can remember that molarity is found by dividing moles by volume, or if you have molarity and volume and want moles, etc.

It'd be easier if I drew it but my scanner is dead.

 

[lolokay]

i understand it now..my formula was wrong. I got another question.

a crystal of table sugar (sucrose, C12H22O11) weighs 1.4mg. How many molecules are in it? How many atoms in total are present?

i worked out the number of molecules being 2.46 x 10^18 but how do you work out the number of atoms present? Would the Molar Mass be the addition of the RAM and you substitute that in place of the RMM as used in the first part of the question?

1 molecule = 12 + 22 + 11 = 45 atoms, so multiply by 45 (so 1.108*10^20)