2 C 2 H 6

TOPIC: Predicting amounts of reagents needed or
amounts of products made
Do Now:
Coefficients in Chemical Equations
• Numbers in front of formulas called coefficients
– Microscopic: Coefficients represent numbers of
individual atoms or molecules
– Macroscopic: Coefficients give mole ratios!
– Moles  connected to mass
Coefficients in Balanced Equations
MOLE-MOLE word problems: convert from
moles of one substance to moles of another
substance
2 C2H6 + 7 O2  4 CO2 + 6 H2O
How much CO2 will be produced if 4 moles
of C2H6 are consumed?
1) Start with balanced chemical equation
2) Problem will ask how many moles/liters given will
yield certain amount something else
3) Use a proportion to compare old mole/liter amounts
to the new mole/liter amounts
How much CO2 will be produced if 4 moles of
C2H6 are consumed?
2 C2H6 + 7 O2  4 CO2 + 6 H2O
= X moles CO2
4 moles C2H6
X = 8 moles CO2
How much H2O will be produced in the
combustion of 5 moles of C2H6?
2 C2H6 + 7 O2  4 CO2 + 6 H2O
X moles H2O
5 moles C H
=
2
6
X = 15 moles H2O
How much oxygen will react with 5 moles of
C 2H 6?
2 C2H6 + 7 O2  4 CO2 + 6 H2O
5 moles = X moles
X = 17.5 moles O2
Gas-Phase Equations
• coefficients in equations represent
ratio of volumes of gases involved in rxn
• volume-volume word problems:
rxns where ALL reactants & products are gases
• volume unit (liter/milliliter) doesn’t matter as long as
constant throughout
How much hydrogen gas will react with 15 liters
of nitrogen gas?
1
N2(g) + 3_________
H2(g)
_________
=
15 L N2(g)
X L H2(g)
X = 45 L H2(g)
 2 NH3(g)
Mass-Volume Problems
• REMINDER: 1 mole any gas occupies
same volume as any other gas (At STP =
22.4 L/mol)
• REMINDER: 1 mole = gram formula mass
–CO2 = 12 + 16 + 16 = 44 g/mol
What volume of Cl2(g) measured at STP is
produced when 7.65g HCl (aq) reacts with
MnO2
MnO2(s) + 4HCl(aq)  MnCl2(aq) + Cl2(g) + 2H2O(l)
1. CONVERT TO MOLES
7.65g HCl
mol
= .2125 mols HCl
36 g HCl
2. Set up ratio
4 = 1
.2125
= .053125 mols Cl2
X
3. Convert Moles to Liters
.053125 mols Cl2
22.4 L = 1.19 L
1 mol
What mass of NH3(g) is produced when
2.15L of H2(g) measured at STP reacts?
3H2(g) + N2(g)  2NH3(g)
1. Set up Ratio
3 =
2.15
2. Convert L to Moles
1.43 L
mol
2
X
= .064 mol NH3
22.4 L
3. Convert Moles to grams
.064 mol 17 g
= 1.09 g
mol
= 1.43 L NH3