CHEMICAL ENGINEERING: Chemistry - Molecular weight, molar mass, molecular mass, moles, Avogadro's number, molecules

Molecular Weight (also called molecular mass or molar mass)

- is the sum of the atomic weights of all the atoms in a molecule.

- it has a unit of amu. One atomic mass unit (1 amu) is 1/12 the mass of the carbon-12 isotope, which is assigned the value 12.



water: H2O

1 molecule of water H2O = 2 atoms of hydrogen + 1 atom of oxygen

components:

2 H * 1 amu = 2 amu
1 O * 16 amu = 16 amu

MW of water = sum of components

MW of water = 2 amu + 16 amu

MW of water = 18 amu



dry air:

MW of air = 29 amu



Moles

A mole (mol) of any substance is the amount of that substance that contains Avogadro's number of atoms or molecules.  Avogadro's number is defined as the number of carbon atoms in 12 g of 12C.  It has a value of 6.022 x10^23 molecules/mol.

A mole of a substance is Avogadro's number of that substance.


n = N/Na

n = m/MW

m = n * MW


where:

n = number of moles

m = mass of substance

MW = molecular weight of substance

N = number of molecules

Na = Avogadro's Number, 6.022 x10^23 molecules/mol




Avogadro's Number

- the number of atoms needed such that the number of grams of a substance equals the atomic mass of the substance, 6.022 x10^23 /mol

- an Avogadro's number of substance is called a mole.

- for example, a mole of carbon-12 atoms is 12 grams, a mole of hydrogen atoms is 1 gram, a mole of hydrogen molecules is 2 grams




Units of molar mass --> g/mol

The most common unit of molar mass is g/mol because in that unit the numerical value equals the average molecular mass in units of u.



1 mole of Water H2O


average atomic mass of Hydrogen =  1 u

average atomic mass of Oxygen = 16 u

molecular mass of water = (2 * 1 u) + 16 u = 18 u

Thus,

1 mole of water has a mass of 18 grams.




Problem 1:

Ten kg of Carbon dioxide has a volume of 998 L at a pressure of 200 kPa. Determine the number of moles of CO2 present. 

find:

n = number of moles of CO2


given:

m = 10 kg = 10,000 g


solution:

Mw of CO2 = 12 + 2*16

MW of CO2 = 12 + 32

MW of CO2 = 44 g/mol


n = m/MW

n = 10,000 g/44 g/mol

n = 227.3 mol




Problem 2:

How many molecules are present in 5 moles of H2O?


find:

N = number of molecules


given:

n = 5 moles


solution:

n = N/Na

5 = N/6.022 x10^23

N = 5 * 6.022 x10^23

N = 30 x10^23

N = 3.0 x10^24 molecules