chemistry - need help.....?

Question

1- calculate the mass of 1.21 x 10^24 atoms of Pb

2- calculate the number of molecules in 757g of C3H8

3- on average the adult human body is comprised of 60% water. how many water molecules are in your body? Mass - 56kg.

if you can answer any of these questions i would be very grateful but please show me how u work them out.

Answer 1

1: Take the mass of 1 atom of Pb (in grams), multiply it by 1.21 x 10^24.

The mass of 1 atom of Pb (grams), is its Relative Atomic Mass (207.2) divided by Avogadro's number (6.02 x 10^23).

2. The Relative Molecular Mass of C3H8 is the sum of the Relative Atomic Masses of its atoms: C = 12, H = 1
So, RMM = 12x3 + 1x8 = 44

So, the MASS OF ONE MOLECULE OF C3H8 (grams), is 44 divided by Avogadro's number again: 44/ (6.02 x 10^23) = 7.4 x 10^(-23)
Then, the NUMBER OF MOLECULES OF C3H8 in 757g is just 757 divided by the MASS OF ONE MOLECULE:
757/ 7.4 x 10^(-23) = 1.02 x 10^25

3. The mass of water in the body is 60% of 56 kg = 56 x 60/100 = 33.6 kg. 33.6 kg = 33600 g (always check your units).
Now, the relative molecular mass of a water molecule, H2O is the mas of 2 x H + 1 x O = 2 x 1 + 1 x 16 = 18. Now, if the RMM is 18, divide by Avogadro's number (6.02 x 10^23) to get the MASS OF 1 MOLECULE OF H2O (grams): = 18/ 6.02 x 10^23 = 2.99 x 10^(-23)
Now, just divide 33600 by 2.99 x 10^(-23).

The key to doing all these questions, is understanding that the atomic mass number (as it appears on periodic tables) divided by Avaogadro's number (6.02 x 10^23), always gives you the mass of the individual atom, in grams. To put it another way, 6.02 x 10^23 atoms of atomic mass 'M', weighs 'M' grams. (The quantity 6.02 x 10^23 is called a MOLE. Therefore, you will sometimes see relative atomic masses or molecular masses expressed with the units: grams per mole).
i hope that's clear.

Answer 2

1.
Divide accordingly
1.21 x 10^24 atoms is about 4 mols of atoms. (1 mol=3 x 10^23)
So to find mass, use molar mass x no. of mols,
4 x 207 = 828g

2.
Molecular mass of C3H8 = 12 x 3 + 8 = 44
No. of mols = mass / molecular mass
...................= 757 / 44
...................= 17.2
Number of molecules = 17.2 x 6 x 10^23
....................................= 1.032 x 10^ 25

3.
60% of 56kg = 33.6kg = 33600g
Number of molecules = mass / molar mass
....................................= 33600 / 18
................................... =1866.7