You probably know the concept of density from physics. Density is how much a substance weighs in relation to its volume. Analogous to density, there is the concept of chemistry in chemistry molar masswhich indicates how much a substance weighs in relation to the number of its particles.
Molar mass definition
Every compound has a specific mass for a certain number of particles. Their ratio of mass to amount of substance is the molar mass M. Accordingly, it is calculated as the quotient of mass m and amount of substance n and has the unit gmol, so the formula is as follows:
M = mn
The amount of substance n is given in the unit mol and 1 mol corresponds to approximately 6,022 * 1023 particles of a compound. In other words, n corresponds to the number of particles. This means that you can use the molar mass M to calculate how many particles of a compound you need to get a certain mass. Please note that each compound has a specific molar mass because each compound is made up of different and/or different numbers of atoms, with the atoms themselves also having different weights. By the way, the number 6,022 * 1023 corresponds to the Avogadro’s constant NA with the unit 1 mol. If you multiply the Avogadro constant by the amount of substance in mol of a substance, you get the number of particles N:
N = NA × n
But why do atoms have different weights? According to Bohr’s atomic model, atoms are made up of an atomic nucleus and an atomic shell. The nucleus contains protons and neutrons, which have approximately the same weight and are much heavier than the electrons in the atomic shell. It is the number of protons that determines which element the chemist is talking about. For example, an atom with only one proton in the nucleus is a hydrogen atom, an atom with six protons is a carbon atom.
The different numbers of protons result in the different masses of atoms. In the same way, the same element can have so-called isotopes, where the number of protons in all isotopes is the same, but the number of neutrons is different. We are then talking about the same element, but isotopes have just as different masses.
You can find out more about this topic in the articles on Bohr’s atomic model and isotope.
Calculate molar mass
Of course, before you can use molar mass in calculations, you need to know how to calculate its value for compounds. This is easy to understand when you consider that the molecular formula of compounds indicates which atoms and how many of these atoms make up a compound. This means that the molar mass of a compound is the sum of the molar masses of its atoms. The molar mass of each atom in the periodic table is given on each element card in the upper right corner.
You are to determine the molar mass of the compound CH4N2O. The molecular formula tells you that the compound consists of one carbon atom, four hydrogen atoms, two nitrogen atoms and one oxygen atom.
If there is no subscript on an element symbol, it means that the atom occurs only once. You could also take place CH4N2O alternatively C1H4N2Write O1, which is unusual.
After you have determined the atomic number of the elements contained, you must read off the corresponding molar mass for each element from the periodic table. Then you would get:
C
H
N
O
12 gmol
1 gmol
14 gmol
16 gmol
Now all you have to do is multiply the molar masses you found out for each element by the number of atoms and add all the values to get the molar mass of the compound CH4N2O to obtain.
M(C1H4N2O1) = 1×M(C) + 4×M(H) + 2×M(N) + 1×M(O) = 12 gmol + 4 gmol + 28 gmol + 16 gmol = 60 gmol
The molar mass is standardized to the so-called atomic mass unit «u». 1 u corresponds to a mass of 1.66 * 10-27 kg. What is meant by normalization is that the molar mass you read off the periodic table is also the mass of a single atom of that element.
If you read off the molar mass of about 22.99 gmol for sodium, it also means that exactly one sodium atom has a mass of 22.99 u or 22.99 * 1.66 * 10-27 kg ≈ Has 38.16*10-27kg. The value already mentioned was chosen for the atomic mass unit u because this results in approximately integer atomic masses in u for as many known elements as possible, which in turn simplifies calculations. 1 u corresponds to one twelfth of the mass of the carbon isotope C612.
The molar mass in exam questions
On the one hand, you have to be able to calculate the molar mass of a compound using the periodic table in an exam. However, you must also be able to use the molar mass to calculate the amount of substance or the number of particles in a compound if the mass is given in g. It is important to understand that the molar mass makes it possible to obtain the amount of substance in moles from the mass of a compound in g and vice versa. For this reason, a corresponding calculation should be presented in the following example, also including the Avogadro constant.
You look at the two connections Cl2 and C2H4OH and their molar masses M(Cl2) ≈ 71 gmol andM(C2H4OH) ≈ 46 gmol. How many particles Cl2 and C2H4OH you need each to reach a mass of 500g? To do this, you convert the formula for the molar mass to n, since a number of particles is required:
n = mM
For the mass m you enter 500g for both connections. You only have to use different values for the molar mass M for each compound, because each compound has a molar mass that is specific to it.
n(Cl2) = mM(Cl2) = 500 g71 gmol ≈ 7.04 mol n(C2H4OH) = mM(C2H4OH) = 500 g46 gmol≈10.87 mol
You can also use the calculated amount of substance in mol and the Avogadro constant to calculate the number of particles using the Avogadro constant:
N(Cl2) = 6.022×1023 1mol×7.04 mol ≈ 42.4*1023N(C2H4OH) = 6.022×1023 1mol×10.87 mol ≈ 65.46*1023
Molar mass – the most important thing
- The molar mass of a compound is the ratio of mass to amount of substance.
- The molar mass has the unit gmol and calculated according to the formelectrical M = mnwhere m is the mass in g (grams) and n is the amount of substance in moles.
- The molar mass of atoms can be found on the periodic table.
- The molecular formula of a compound tells you how many atoms are in the bond from which element. This allows the molar mass of a compound to be calculated by multiplying the molar masses of the elements present by their number of atoms and then adding all the values.