Calculating the number of molecules in water
2020-04-15 - Kiss J Gabor
Drops in the water
Chemical formula of water
To calculate the number of molecules in, you need to know the chemical formula of water. There are two atoms of hydrogen and one atom of oxygen in each water molecule. They make up the formula H2O.
Molar mass of water
Determine the molar mass of water. Do this by adding up the mass of hydrogen atoms and oxygen atoms in a mole of water. One has to look the atomic mass of hydrogen and oxygen on the periodic table. The mass of hydrogen is 1,008 g/mol and the mass of oxygen is 15,99 g/mol. The mass of a mole of water can be calculated as follows:
Mwater = 2 × MH + MO
Mwater = 2 × 1,008 + 15,99Mwater = 18,015 g
The Avogrado’s Number
Avogadro’s number tells us the number of molecules of water per mole of water:
NA = 6,022 x 1023 1/mol
Density of water
Use the density of water to determine the mass of water per unit volume. The value used in calculations is:
1,00 g/ml .
Determine the number of molecules in
a.) 1 mole, i.e. 18,015 g, i.e. 18,015 ml of water
n = 6,022 x 1023 molecules
i.e. - id est
b.) 1 g, i.e. 1 ml, i.e. 1 cm3 of water
n = 6,022 x 1023 / 18,015 = 3,343 x 1022 molecules
c.) 1000 g, i.e. 1 l, i.e. 1 dm3 of water
n = 1000 x 3,34322 = 3,343 x 1025 molecules
d.) 0,001 g, i.e. 0,001 ml, i.e. 1 mm3 of water
n = 0,001 x 3,34322 = 3,343 x 1019 molecules
and so on...