by Cesare Curti
|
To transform daltons into grams is like to
transform centimetres into meters.
Number 100 is the equivalent in centimetres of the meter just like
Avogadro number is the equivalent in daltons of the gram |
|
Used symbols |
|
|
|
|
|
Avogadro number |
N |
|
dalton |
u |
|
gram |
g |
|
carbon 12 |
12C |
|
|
|
The masses of atoms commonly are expressed in atomic mass unit or dalton [1]. The dalton is defined
in this way: the dalton is the
twelfth part of the mass of an atom of 12C
From this definition (later called definition
1) comes down immediately that the mass of an atom of 12C is 12 u.
More equivalent definitions of Avogadro number
can be given. One of they is
following: N is the number of atoms
contained in 12 g of 12C
From this last definition (later called
definition 2) it comes down immediately that 12 g of 12C contain N
atoms.
Using the two previous definitions it is easy
to demonstrate that 1 g is equivalent to N u.
It can be reasoned in this way:
12 g of 12C contain N atoms of 12C (for definition
2) every one of which have a mass of 12 u (for the definition 1). It can
therefore be written:
12 g of atoms of 12C = 12N u of
atoms of 12C
The mass
is independent from the species of the substances and simplifying:
1 g = N u
Therefore it has been demonstrated that the Avogadro
number is the equivalent in dalton of the gram. Consequently 1/N is the equivalent in grams of the dalton.
Gram and dalton are both mass measure unit and
therefore the Avogadro number, like over defined, is a pure number or if
one is preferred, non-dimensional properties. The National Institute of
Standard and Technology (NIST) defines instead the Avogadro constant NA [2]:
NA = 6.022 141 99(47) 1023 mol-1
N and NA have the same numerical
value, experimentally determinable by means of many independent ways.
Later on we will use:
N = 6.022 1023 u g-1
Questions
and answers
Question: twenty-five grams to how many daltons
correspond?
Answer: twenty-five grams correspond to 25 g N u g-1 = 25 g 6.022
1023 ug-1 = 1.506 1025 u
Question: one million of daltons to how many grams
correspond?
Answer: one million of daltons correspond to 106
u N-1 u-1g = 106 u 6.022-1 10-23
u-1g = 1.661 10-18 g
Question: which is the mass in grams of 1 atom of 12C?
Answer: the mass in grams of a 12C atom
is 12 u N-1 u-1g
= 12 u 6.022-1 10-23 u-1g =
1.993 10-23 g
.
Bologna,
giovedě 25 gennaio 2001