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Mole-Mass Relationships
 Suppose
Mole-Mass Relationships
you need 3.00 mol of NaCl
for a laboratory experiment
 How can you measure this amount?
 What mass in grams represents
3.00 mol of NaCl?
 Use the molar mass to convert
between the mass of a substance and
the number of moles of a substance
Problem
Mole-Mass Relationships
 Aluminum
 The
molar mass of NaCl is
58.5 g/mol:
 When
you measure 176 g of NaCl on
a balance, you are measuring
3.00 moles of NaCl!
satellite dishes are
resistant to corrosion because the
aluminum reacts with oxygen in the
air to form a coating of
aluminum (III) oxide (Al2O3)
 The tough Al2O3 coating prevents
any further corrosion of the pure Al
 What is the mass of 9.45 mol of
aluminum oxide?
Solution
 Known:
 9.45
mol Al2O3
 Unknown:
g
Al2O3?
 Strategy:
 mol
Solution
 To
solve this problem you need to
know the molar mass of Al2O3
 What is the molar mass of Al2O3?
 (2)(26.9815
amu) + (3)(15.9994 amu)
= 101.9612 amu = 101.9612 g/mol
 ~ 102 g/mol
→ mass
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Solution
 Always
check that the answer makes
sense!
 We are starting with ~ 10 mol Al2O3
 The molar mass of Al2O3 is
~ 100 g/mol
 Therefore, the answer should be
approximately 1000 g
Problem
 Suppose
that 10.0 g of sodium
sulfate (Na2SO4) are produced in
an experiment
 How many moles of sodium
sulfate does this represent?
Problem
Solution
compute the molar mass of
Na2SO4
 The molar mass of Na2SO4 is
142.1 g/mol
 First,
 When
iron is exposed to air, it
corrodes to form rust
 Rust is iron (III) oxide (Fe2O3)
 How many moles of iron (III)
oxide are contained in 92.2 g of
pure Fe2O3?
Solution
 Known:
 92.2
g Fe2O3
 Unknown:
 mol
Solution
the molar mass of Fe2O3
 The molar mass of Fe2O3 is
159.6 g/mol
 Compute
Fe2O3?
 Strategy:
 mass
→ mol
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Solution
 Because
the sample mass (~90 g)
is slightly larger than the mass of
0.5 mol Fe2O3 (~160 g), the
answer should be slightly larger
than 0.5 mol
 0.578 mol Fe2O3 – it is
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