ANSWERS--Balancing Equations Examples/Practice
1. ___ Cu2O + ___ Cu2S → ___ Cu + 1 SO2
2 Cu2O + ___ Cu2S → ___ Cu + 1 SO2 (add 2 to balance the O’s)
(Don’t pick either reactant as your starting
coefficient because both have Cu in them.
Never pick an element (i.e. Cu) to start with)
2 Cu2O + 1 Cu2S → ___ Cu + 1 SO2 (add 1 to balance the S’s)
2 Cu2O + 1 Cu2S → 6 Cu + 1 SO2 (balance the element last)
2. ___ HCl + 1 Al(OH)3 → ___ AlCl3 + ___ H2O
___ HCl + 1 Al(OH)3 → 1 AlCl3 + ___ H2O (balance the Al’s)
(most complex formula [with 2 of 3 atoms only
in one formula on that side] chosen to start)
3 HCl + 1 Al(OH)3 → 1 AlCl3 + ___ H2O (balance the Cl’s; [could have done O’s first])
3 HCl + 1 Al(OH)3 → 1 AlCl3 + 3 H2O (balance the O’s)
3. 1 CH3OH + ___ O2 → ___ HCHO + ___ H2O (started with most complex)
1 CH3OH + ___ O2 → 1 HCHO + ___ H2O (balance the C’s)
1 CH3OH + ___ O2 → 1 HCHO + 1 H2O (balance the H’s)
1 CH3OH + ½ O2 → 1 HCHO + 1 H2O
(balance the O’s [NOTE: you don’t have “3
O’s on the left” before you add the coefficient
to O2! You only have one committed O! So
adding a ½ “works” to get you a total of two.)
2 CH3OH + 1 O2 → 2 HCHO + 2 H2O (multiply all through by 2)
4. ___ P + ___ Fe2O3 → 1 P4O10 + ___ Fe (Complex-looking formula to start.)
4 P + ___ Fe2O3 → 1 P4O10 + ___ Fe (Balance the P’s)
4P+
10
3
4P+
10
3
Fe2O3 → 1 P4O10 + ___ Fe
Fe2O3 → 1 P4O10 +
20
(Balance the O’s. This is a bit cumbersome with a fraction, but it works!)
Fe (Balance the Fe’s)
3
12 P + 10 Fe2O3 → 3 P4O10 + 20 Fe (Multiply by 3 to get rid of the fractions.)
5. ___ Pb + ___ PbO2 + 1 H2SO4 → ___ PbSO4 + ___ H2O (Pick a complex formula to start)
___ Pb + ___ PbO2 + 1 H2SO4 → 1 PbSO4 + ___ H2O (Balance S’s)
___ Pb + ___ PbO2 + 1 H2SO4 → 1 PbSO4 + 1 H2O (Balance H’s)
___ Pb + ½ PbO2 + 1 H2SO4 → 1 PbSO4 + 1 H2O
½ Pb + ½ PbO2 + 1 H2SO4
→ 1 PbSO4 + 1 H2O (Balance the Pb’s)
(Balance O’s [Note, you do NOT have 6 O’s
on the left before adding a coefficient to
PbO2! You only have 4 committed O’s, so
you can get 1 more by using a ½ .])
1 Pb + 1 PbO2 + 2 H2SO4 → 2 PbSO4 + 2 H2O (Multiply by 2 to get rid of the fractions.)
6. ___ PbO + ___ PbS →___ Pb + 1 SO2
___ PbO + 1 PbS →___ Pb + 1 SO2 (Balance the S’s)
2 PbO + 1 PbS →___ Pb + 1 SO2 (Balance the O’s)
2 PbO + 1 PbS → 3 Pb + 1 SO2 (Balance the Pb’s)
(Lead is in both reactants, so avoid starting with
either of those. Pb is an element and should be
done last. So start with SO2 “by default”.)