The Chapter Story: Calculating the number of Vit-B12 molecules
The amount of 125 m
g of vit-B12 in a tablet is a very small amount compared to a 1-g vit-C
tablet! Find out how many vit-B12 molecules is in that tiny 125 m
g of vit-B12! The formula of vit-B12 is C_{63}H_{88}CoN_{14}O_{14}P.
(The molar mass of vit-B12 is 1355 g/mol.)
Thought process:
mass ---> # mole ---> # molecule
Let’s start with the simple molecule H_{2}O!
The average mass in the unit of amu or molar mass (g/mol) of a representative unit of any pure substance
NaCl has a latticed structure in which each Na^{+} is surrounded by 6 Cl^{–} anions and likewise each Cl^{–} is surrounded by 6 Na^{+} cations with the representative unit NaCl, i.e., one Na^{+} and one Cl^{–}. Its formula mass is 22.9898 (of Na) + 35.453 (of Cl) = 58.443 amu or g/mol.
(The lattice can be seen at http://www.kings.edu/~chemlab/animation/nacl.html.)
How about water and glucose (C_{6}H_{12}O_{6})?
Or a more "general definition" as in the textbook as "The number of atoms (or formula units or molecules) contained in the atomic mass in g (or formula mass in g) of an element (or a compound)".
Molar mass: mass in g of 1 mol of the substance
Examples (see more on Table 4.1)
a) water, 18 g/mol (2 mol H + 1 mol
O = 2 g + 16 g), or mol/18 g
2 mol H atoms react
with 1 mol O atoms to give 1 mol water molecules
(But, remember
"all gases are diatomic, except noble gases." Thus, instead of having 2H
+ O ® H_{2}O,
you MUST write 2H_{2} + O_{2} ®
2H_{2}O.)
b) CO_{2}, 44 g/mol (1 mol
C + 2 mol O = 12 g + 16 g ´
2), or mol/44 g
1 mol C reacts
with 2 mol O (1 mol O_{2}) to give 1 mol CO_{2};
c) A small size protein with ~100 amino acids, ~10,000 g/mol.
The "larger" the molecule, the more the mass is in a mole of the molecule.
Exercises: (Chapter 12, Smith
& Vukovich)
a) What is the weight in grams
of 0.25 mol sucrose (C_{12}H_{22}O_{11})?
The formula mass of sucrose is _______(Y)
g/mol.
Thus, 0.25 mol ´
______ (Y) g/mol = _______ g
(To think algebraically, X mol of
a compound with a formula mass Y (g/mol) has an amount (g) of X (mol)
´ Y (g/mol).)
b) A tablet of an antacid contains 500.0 mg Al(OH)_{3} (What is its electron dot structure?). How many moles of Al(OH)_{3} are in one tablet? How many Al(OH)_{3} units are there? How many moles of Al, O, and H are there?
The formula mass of Al(OH)_{3}
is _______ (Y) g/mol.
500.0 mg = 0.5000 g
0.5000 (g)/Y (g/mol)
= _______ mol Al(OH)_{3}
Each mol Al(OH)_{3} has __
mol Al, __ mol O and __ mol H.
(How many moles (or molecules) of sucrose, water, the 10,000 g/mol protein above, ethanol C_{2}H_{6}O, or CO_{2} are in 0.5 g of each?)
Empirical formula
The simplest whole-number ratio of
the constituents of a compound (which can be determined with elemental
analysis techniques).
Thus, acetic acid (CH_{3}COOH)
has an empirical formula of CH_{2}O, which describes that the ratio
of C:H:O = 2:4:2 = 1:2:1
Examples 4.8
A compound has 36.04% Ca and 63.96%
Cl. Determine its empirical formula.
First, find the percent ratios of each constituent. Then, convert the percent ratios into "moles". Finally, find the simplest ratio of the mole ratio.
36.04% Ca is equivalent to 36.04 g
in 100 g of the compound
63.96% Cl = 63.96 g Cl/100 g compound
(How many grams
of Ca or Cl is in 40 g of the compound?)
Find the molar ratio of Ca:Cl
36.04 g Ca ´
1 mol/40.08 g Ca = 0.8992 mol Ca
63.96 g Cl ´
1 mol/35.45 g Cl = 1.804 mol Cl
Ca:Cl = 0.8992:1.804
= (0.8992/0.8992):(1.804/0.8992) =1.000:2.006
~ 1:2
(Do you get the
same answer if you start with 40 g of material?)
A few more examples can be found at:
http://www.towson.edu/~ladon/empiric.html