Moles to Mass:
By definition, 1 mol of Carbon-12 has a mass of 12g and since all relative atomic masses of elements are relative to the Carbon-12 isotope, the following application is made that:
1 mol of atoms of any element corresponds to the R.A.M. of that element expressed in grams.
Examples:
1 mol of atoms of any element corresponds to the R.A.M. of that element expressed in grams.
Examples:
- 1 mol of Li atoms has a mass of 6.9g since its R.A.M. is 6.9
- 1 mol of Na atoms has a mass of 23.0g since its R.A.M. is 23.0
- Therefore, 2 mol of Na atoms has a mass of 46.0g (2 x Mr(Na))
General Formula for Reacting Masses:
m = Mass (g) n = Number of moles (mol) Mr = Relative molecular mass
The formula relating mass, m, number of moles, n, and the relative molecular mass, Mr is:
m = nMr
Or re-arranged to:
n = m / Mr
The formula relating mass, m, number of moles, n, and the relative molecular mass, Mr is:
m = nMr
Or re-arranged to:
n = m / Mr
Example:
A sample of NaOH weighs 10.0g. Calculate the amount, in mol, of NaOH present.
n = m / Mr
m(NaOH) = 10.0g
Mr(NaOH) = 23.0+16.0+1.0
n(NaOH) = m(NaOH) / Mr(NaOH) = 10.0g / (23.0+16.0+1.0) = 0.250 mol
A sample of NaOH weighs 10.0g. Calculate the amount, in mol, of NaOH present.
n = m / Mr
m(NaOH) = 10.0g
Mr(NaOH) = 23.0+16.0+1.0
n(NaOH) = m(NaOH) / Mr(NaOH) = 10.0g / (23.0+16.0+1.0) = 0.250 mol
Calculations involving the Avogadro constant:
P = number of particles n = number of moles L = Avogadro constant
The formula relating moles, n, the Avogadro constant, L, and the number of particles, P is:
P = nL
The formula relating moles, n, the Avogadro constant, L, and the number of particles, P is:
P = nL
Example:
Calculate the number of water molecules, H2O, in 25.0g of water.
n = m / Mr
m(H2O) = 25.0g
Mr(H2O) = 1+1+16
n(H2O) = 25.0g / 18.0 = 1.39 mol
P = nL
Number of water molecules = n x L = 1.39 mol x (6.023 x 10^23 mol^-1) = 8.36 x 10^23 water molecules
Calculate the number of water molecules, H2O, in 25.0g of water.
n = m / Mr
m(H2O) = 25.0g
Mr(H2O) = 1+1+16
n(H2O) = 25.0g / 18.0 = 1.39 mol
P = nL
Number of water molecules = n x L = 1.39 mol x (6.023 x 10^23 mol^-1) = 8.36 x 10^23 water molecules