# Download Atomic mass unit (AMU) The masses of protons and neutrons are

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```Atomic mass unit (AMU)
The masses of protons and neutrons are nearly equal (1.0073 and 1.0087 amu respectively).
A mole is the amount of matter that contains as many objects (atoms, molecules, or other objects) as
the number of atoms in 12 grams of 12 C . This number has been determined through experimentation
to be 6.022 ! 10 23 , and is known as Avogadro’s number.
A mole of atoms, a mole of molecules, or a mole of anything all contain Avogadro’s number of these
objects:
1 mol 12C atoms = 6.022 ! 10 23 12C atoms
1 mol H 2O molecules = 6.022 ! 10 23 H 2O molecules
Ex: Convert 1.8 ! 10 –11
J
J
to
, where U = Uranium.
U nucleus
mole of U
%
J
J
"
%"
–11
23 U nuclei
13
\$# 1.8 ! 10 1 U nucleus '& \$ 6.022 ! 10 mole of U ' = 1.08 ! 10 mole of U
#
&
The mass of a single atom of an element [amu] is numerically equal to the mass [g] of 1 mol of that
element, regardless of the element.
1 atom of
12
C has a mass of 12 amu ! 1 mol
12
C has a mass of 12 g.
Molar mass
The molar mass of an element is the atomic weight in grams per mole, i.e. the molar mass [g/mol] of
any substance is always numerically equal to its formula weight [amu]
e.g.
1 H2O molecule has a mass of 18.0 amu ↔ 1 mol H2O has a mass of 18.0 g
1 O2 molecule has a mass of 16.0 amu ↔ 1 mol O2 has a mass of 16.0 g
Q: How many moles of water are in a liter of water, if 1 liter = 1 kilogram water.
A: Atomic weights:
Hydrogen: 1.008 amu
Oxygen: 16.00 amu
Formula weight = 1.008 amu + 1.008 amu + 16.00 amu = 18.016 amu
! 1 mol \$
1000 g #
= 55.55 moles of water
" 18.0 g &%
References
Chemistry, the Central Science. Brown et al., Pearson
```
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