Mole and molecular weight
Basic chemistry
Content created by REAL intelligence since 2016
What is a mole?
A mole is a number, just like a dozen, a score or a gros. A mole is also called Avogadro's number and in chemistry and physics written as either NA or L. The latest value for NA is 6,02214179 · 1023. This value is from 1996.
The reason we can talk about the latest value, and thus the reason you can find other values in older litterature, is due to the origin or the number. A mole is defined as the number of carbon atoms in 12,0 grams of 12C. For practical reasons, we do not have the exact number to the 23rd decimal, but the development of new techniques for measuring through the years, has made us wiser/more accurate, and the adjustment of Avogadro's number is a reflection of this updated knowledge.
So basically, a mole is just a number. A very large number, but just a number.
The reason we can talk about the latest value, and thus the reason you can find other values in older litterature, is due to the origin or the number. A mole is defined as the number of carbon atoms in 12,0 grams of 12C. For practical reasons, we do not have the exact number to the 23rd decimal, but the development of new techniques for measuring through the years, has made us wiser/more accurate, and the adjustment of Avogadro's number is a reflection of this updated knowledge.
So basically, a mole is just a number. A very large number, but just a number.
What is molecular weight?
Molecular weight is the weight of one mole of a chemical compound or atom, depending on what we are working with. Molecular weight is essentially the same as atomic weight, except atomic weight refers to the free single atoms. The value/number is the same.
The unit for molecular weight is gram per mole, g/mol, and designated M. The value normally given for elements is the average value for naturally occurring isotopes. For elements not occurring naturally, it is the average for the known isotopes. Therefore the molecular weight for carbon is seen as 12.011 g/mole instead of 12.000 g/mole. That is the contributions from naturally occuring isotopes such as 13C and 14C, increasing the average.
Molecular weights for elements is something you have to look up. Like Avogadro's number, you can find different values the litterature, due to improvements in equipment and measuring techniques, and we have gathered more information about the natural composition of isotopes. The differences is these are minor, but if you are not aware of the differences and the reasons for this, it can cause some confusion when looking up the values.
The unit for molecular weight is gram per mole, g/mol, and designated M. The value normally given for elements is the average value for naturally occurring isotopes. For elements not occurring naturally, it is the average for the known isotopes. Therefore the molecular weight for carbon is seen as 12.011 g/mole instead of 12.000 g/mole. That is the contributions from naturally occuring isotopes such as 13C and 14C, increasing the average.
Molecular weights for elements is something you have to look up. Like Avogadro's number, you can find different values the litterature, due to improvements in equipment and measuring techniques, and we have gathered more information about the natural composition of isotopes. The differences is these are minor, but if you are not aware of the differences and the reasons for this, it can cause some confusion when looking up the values.
Calculations with molecular weight
Molecular weights for chemical compounds are, contrary to the elements, something you calculate, if you know the formula. For unknown substances you do it the other way around and determine the molecular weight experimentally and calculate backwards to the formula. This can be a both complicated and time consuming affair, involving several analytical methods, and it is something we will address under analytical chemistry.
Molecular weight is simple addition. The molecule H2 consists of 2 hydrogen atoms, so
This also applies for combinations of elements such as H2SO4:
Complex compounds is exactly the same, you just have to keep track of more values, e.g. Ca5(PO4)3F (fluorapatite):
For ions and complexes you do the calculations just like any other chemical compound. Charges changes nothing on the molecular weight, so for instrance the nitrate ion (NO3−) is:
Molecular weight is simple addition. The molecule H2 consists of 2 hydrogen atoms, so
| M(H2) | = 2 × M(H) = 2 × 1.0079 g/mole = 2.0158 g/mole |
This also applies for combinations of elements such as H2SO4:
| M(H2SO4) | = 2 × M(H) + M(S) + 4 × M(O) = 2 × 1.0079 g/mole + 32.06 g/mole + 4 × 15.9994 g/mole = 98.07 g/mole |
Complex compounds is exactly the same, you just have to keep track of more values, e.g. Ca5(PO4)3F (fluorapatite):
| M(Ca5(PO4)3F) | = 5 × M(Ca) + 3 × [M(P) + 4 × M(O)] + M(F) = 5 × 40.078 g/mole + 3 × [30.97376 g/mole + 4 × 15.9994 g/mole] + 18.9984 g/mole = 504.302 g/mole |
For ions and complexes you do the calculations just like any other chemical compound. Charges changes nothing on the molecular weight, so for instrance the nitrate ion (NO3−) is:
| M(NO3−) | = M(N) + 3 × M(O) = 14.0067 g/mole + 3 × 15.9994 g/mole = 62.0049 g/mole |
What is it used for?
It is used for something called stoichiometric calculations for chemical reactions. When you look at reactions, you always write the molecular ratio (number of moles reacting), e.g.:
2 H2(g) + O2(g)
2 H2O(g)
Here H2 and O2 react in the ratio 2:1, but since we cannot measure 2 and 1 mole of H2 and O2 respectively, we have to convert the number of molecules to masses, for instance, using the formula m = n · M:
Now we know that we need to weigh off 4 g H2 to 32 g O2 to have the right mixing ratio for a complete reaction.
By using this technique you can calculate things like how much raw material you need for production, the environmental impact for various types of waste management, CO2 emissions, etc., and this is something being done in real life.
2 H2(g) + O2(g)
2 H2O(g)Here H2 and O2 react in the ratio 2:1, but since we cannot measure 2 and 1 mole of H2 and O2 respectively, we have to convert the number of molecules to masses, for instance, using the formula m = n · M:
| m(H2) | = n(H2) · M(H2) = 2 mole · 2.0158 g/mole = 4.0316 g |
| m(O2) | = n(O2) · M(O2) = 1 mole · 31.9988 g/mole = 31.9988 g |
Now we know that we need to weigh off 4 g H2 to 32 g O2 to have the right mixing ratio for a complete reaction.
By using this technique you can calculate things like how much raw material you need for production, the environmental impact for various types of waste management, CO2 emissions, etc., and this is something being done in real life.