The formula mass
Moles of
compounds
We know how to calculate a mole of any element from its average atomic mass.
However, most matter exists in compounds. What about compounds? Most of chemistry
deals with compounds. How do we calculate moles of a compound?
The formula
mass
The answer is to use the chemical formula. The formula
mass is the mass of one mole of a compound with a given
chemical formula. As an example, start with water (H2O).
What is the mass (in grams) of one mole of water?
1. The chemical formula says one mole of water contains
two moles of hydrogen and one mole of oxygen.
2. Add up the mass of two moles of hydrogen (2 g) and one
mole of oxygen (16 g).
3. The result (18 g) is the mass of 1 mole of water.
What is the mass of 1 mole of methane (CH4)?
Asked:
Given:
The mass of 1 mole of methane
Methane (CH4) contains 1 carbon (C) and 4 hydrogen (H) atoms
Relationships: The formula mass is the sum of the atomic masses for each atom in
the compound.
Solve:
4H + C = 4(1.0079) + 12.011 = 16.04 g per mole of CH4
Answer:
One mole of methane (CH4) has a mass of 16.04 g.
How many moles are in 100 grams of water (H2O)?
Asked:
Given:
The moles in 100 g of water
Water (H2O) contains 2 hydrogen (H) atoms and 1 oxygen (O) atom.
Relationships: The formula mass is the sum of the atomic masses for each atom in
the compound.
Solve:
2H + O = 2(1.0079) + 15.999 = 18.015 g per mole of H2O
1 mole
100 g × ⎛⎝ ---------------------⎞⎠ = 5.55 moles
18.015 g
Answer:
100 g of water (H2O) contains 5.55 moles.
formula mass: the mass of one mole of a compound with a given chemical formula.
A NATURAL APPROACH TO CHEMISTRY
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Section 2.2 Molecules and Compounds
Solving problems with moles
Converting back and forth between grams and moles is a very useful skill in chemistry.
This is because we weigh substances in grams, but we need to know how many moles
there are in order to determine the number of atoms or molecules in that mass. The
periodic table allows us to obtain the formula mass for any compound with which we
might be experimenting in the lab. You will find unit cancellation to be very important
when you are doing this type of calculation. Keeping track of your units helps you to
eliminate simple mistakes. To convert from grams to moles we divide by the molar mass.
How many moles are in 25.0 g of copper oxide (CuO)?
Asked:
Moles of CuO
Given:
25.0 g of CuO
Relationships: The formula mass for CuO is 63.55 + 15.999 = 79.55 g/mole.
Solve:
Answer:
1 mole CuO
25.0 g CuO × ------------------------------ = 0.314 moles of CuO
79.55 g CuO
25.0 g of CuO contains 0.314 moles of copper oxide (CuO).
How many grams are in 2.300 moles of butane (C4H10)? Butane is used as a
lighter fluid in disposable lighters.
Given:
2.300 moles of C4H10
Relationships: The formula mass for C4H10 is 4(C) + 10(H)
= (4 × 12.011) + (10 × 1.0079) = 58.123 g/mole.
Solve:
58.123 g C 4 H 10
2.300 moles C 4 H 10 × ---------------------------------------- = 133.7 g C 4 H 10
1 mole C 4 H 10
Answer:
133.7 g are in 2.300 moles of butane (C4H10).
How many moles are in 30.0 g of baking soda (NaHCO3)?
Given:
30.0 g of NaHCO3
Relationships: The formula mass for NaHCO3 is
22.99 + 1.0079 + 12.011 + 3 × 15.999 = 84.01 g/mole.
Solve:
Answer:
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1 mole
30.0 g NaHCO 3 × ------------------------------------------- = 0.357 moles
84.01 g NaHCO 3
30.0 g are in 0.357 moles of baking soda (NaHCO3).
A NATURAL APPROACH TO CHEMISTRY
Using Avogadro’s number
We can use Avogadro’s number to tell us how many atoms are in a given mass of a metal,
such as lead (Pb).
How many atoms of lead (Pb) are in a lead pipe that weighs 105 g?
Given:
Atoms in 105 g of Pb
Relationships: The atomic mass of lead is 207.2 g/mole.
23
23
1 mole Pb 6.022 x 10 atoms Pb
105 g Pb × ------------------------- × ------------------------------------------------------ = 3.05 × 10 atoms Pb
1 mole Pb
207.2 g Pb
There are 3.05 × 1023 atoms of Pb in a lead pipe that weighs
105 g.
Answer:
Sometimes we want to know how many atoms of an element are in a given mass of a
compound, as illustrated in the following example:
How many oxygen atoms are in 200.0 g of glucose (C6H12O6)?
Given:
200.0 g of C6H12O6
Relationships: The molar mass of glucose is 6(C) + 12(H) + 6(O)
= 6(12.011) + 12(1.0079) + 6(15.999) = 180.15 g/mole.
Solve:
First we find how many moles are in 200.0 g:
1 mole C H O
6 12 6
200.0 g C H O × ------------------------------------------------ = 1.110 moles C H O
6 12 6 180.15 g C H O
6 12 6
6 12 6
Next we find how many molecules are contained in 1.11 moles:
23
6.022 × 10 molecules C 6 H 12 O 6
23
1.110 mole C 6 H 12 O 6 × ------------------------------------------------------------------------------------ = 6.684 × 10 molecules C 6 H 12 O 6
1 mole C 6 H 12 O 6
Then we find how many oxygen atoms are contained in 1.110 moles
of glucose:
6.684 × 10
23
Answer:
A NATURAL APPROACH TO CHEMISTRY
6 O atoms
molecules C 6 H 12 O 6 × ------------------------------------------------------------- = 4.010 × 10
1 molecule C H O
6 12 6
24
atoms of O
There are 4.010 × 1024 atoms of O in 200.0 g of glucose (C6H12O6).
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