Best Answer:
It's been a while since I did chemistry but I'll give it a go (maybe someone else can verify my answer - or by the time I get it done others might have already answered it).

Note that the numbers you use might be slightly different from what I'm using here because I'm using average atomic weights and not taking the isotopes into account. If your teacher hasn't mentioned isotopes then they were probably using figures similar to what I'm using below.

The ^ sign is used to denote "to the power of" because I can't use superscript here.

(a)

All you have to do is work out the atomic mass of the molecule and then multiply that by the number of moles.

For the first one, if you look on the periodic table,

the atomic weight of manganese is 54.938045u.

the atomic weight of sulphur (English spelling - sulfur in American) is 32.065u

and the atomic weight of oxygen is 15.9994u. Now you can see that there are 4 atoms of oxygen so the weight of the oxygen is 4 x 15.9994 = 63.9976u.

Now add them all together: 54.938045 + 32.065 + 63.9976 = 151.000645u.

This figure is the total atomic weight of the molecule. If it was asking for the weight of 1 mole, then this is the figure.

Since it's asking for 0.64 mol, you multiply the molecule weight (151.000645) by 0.64 to get 96.6404128u which is the answer (just change the units to grams, the number stays the same).

I think that's right.

(b)

For the next one, we'll first work out the weight of the molecule using the same method as the previous answer:

Iron = 55.845u

Chlorine = 35.453u

Oxygen = 15.9994u

Now there are 3 lots of the perchlorate molecules (ClO4) and 4 lots of the oxygen atom in each perchlorate molecule. So that's 3 chlorine and 12 oxygens. So we multiply the weights above by the number of atoms of each.

Iron = 1 atom x 55.845u = 55.845u.

Chlorine = 3 atoms x 35.453u = 106.359u.

Oxygen = 12 atoms x 15.9994u = 191.9928u.

Now add them all together to get the total weight of the molecule:

55.845+106.359+191.9928 = 354.1968u.

As mentioned before, this is the number of grams in 1 mole. We need to know how many moles given 15.8 grams. 15.8 is less than 354.1968 so we would expect the answer to be less than 1 mole. So we take 15.8 and divide it by 354.1968 to get 0.04461 mol. We're dividing here rather than multiplying like in the first example because we're going the other way (it's basic mathematical formula manipulation):

weight x mol = grams This was the first example.

grams / weight = mol.

And that's my answer, 0.04461 mol.

(c)

Now this one's more interesting because we're dealing with the weight of just the nitrogen atoms. We could use Avogadro's Constant here if you've learnt that at school. This constant tells us the number of atoms in 1 mole of the substance. So first we work out the number of moles we have and then multiply that by Avogadro's Constant to get the number of atoms.

The weight of the molecule:

Nitrogen = 14.0067 x 2 = 28.0134u

Hydrogen = 1.00794 x 4 = 4.03176u

Oxygen = 15.9994 x 2 = 31.9988u

Total = 64.04396u

This total is the number of grams in 1 mol.

We need to work out the number of moles so we do like in (b).

92.6 / 64.04396 = 1.44588186 mol.

Avogadro's Constant is 6.022 141 79 × 10^23

If we multiply these together, we get the number of molecules that we have. This is 1.44588186 x 6.02214179 x 10^23 = 8.707305573x10^23.

Now because there are 2 nitrogen atoms in each molecule, we multiply the number of molecules by 2 to get the number of nitrogen atoms in total:

8.707305573x10^23 x 2 = 1.7415x10^24 atoms.

It would be good if someone could check my answers or verify the results because it's been a long time since I did chemistry and it's past midnight here. My maths skills go down exponentially after midnight.

Source(s):
http://www.ausetute.com.au/moledefs.html

http://www.webelements.com

http://en.wikipedia.org/wiki/Avogadro_constant

Anonymous
· 1 decade ago

Asker's rating