Question

calculate the # of H atoms contained in 2.3g of C2H5OH.

I just need help understanding why we must divide 2.3 by 46 (which is the molecular mass) for the first step?

below is my worked solution:
2.3g/4.6g = 0.05 mols
6 * 0.050 moles = 0.30 mols
0.3 x 6.01e23 = 1.8066e23

thanks!

Answer 1

@cwrw238 @Preetha or any chemistry people out there??

Answer 2

Firstly, the molecular weight of the molecule is 46 g/mol. This unit (g/mol) means that there is one mole of molecules in 1 g of the compound you are examining.

You have 2.3 g of the compound you are analysing. To figure out how many moles there is in the sample, you divide 2.3 g by 46 g/mol.

(When I do these sums, I always include the units in the equation and let them cancel like x or y in algebra. If you do it this way, you should be left with the correct unit at the end if you have done the sum properly.)

2.3 g / (46 g/mol) = 0.05 mol
Now you know how many moles you have of C2H5OH in the sample.

The next step is to figure out what proportion of the molecule is made of hydrogen and then figure out how many hydrogen atoms there are in the sample as a whole.

There are 6 hydrogens in the molecule with a total weight of 6 g/mol.
Now figure out what percentage of the molecule is hydrogen:
[(6 g/mol)/(46 g/mol)]*100= 14.28% of the molecule is hydrogen.

Now all you have to do is find 14.28% of 0.05 mol to figure out how many moles of hydrogen you have in the sample. -> (14.28%)*(0.05 mol) = 0.00714 mol.
Almost there!

The final step is to calculate how many hydrogen atoms there are in 0.00714 mol.
To do this you simply multiply by avogadros number!
(avodagros number says that there is 6.01e23 molecules in one mole of a pure compound)

-> (0.00714 mol)*(6.01e23 hydrogen atoms/mol) = 42.91e20 hydrogen atoms.

Therefore, there are 42.91e20 hydrogen atoms in 2.3 g of C2H5OH (ethanol).

Answer 3

thank you so much for such a detailed answer!! so in the way I did it, is it wrong to multiply 6 and 0.05moles?