We have 20 g of a certain element that has a half life of 10 minutes. 30 minutes pass. How much is
left of that element?

You classify the equation as single replacement or double replacement. Single replacement has the form: A + BC --> AC + B. Double replacement has the form: AB + CD --> AD + BC.

1.
a. Single replacement
b. Al + Fe₂O₃ --> Al₂O₃ + Fe
c. 2Al + Fe₂O₃ --> Al₂O₃ + 2Fe

2. 
a. Double replacement
b. Pb(C₂H₃O₂)₂ + H₂S --> PbS + HC₂H₃O₂
c. Pb(C₂H₃O₂)₂ + H₂S --> PbS + 2 HC₂H₃O₂

3. 
a. Single Replacement
b. Mg + HCl --> MgCl₂ + H₂
c. Mg + 2HCl --> MgCl₂ + H₂

4. 
a. Single Replacement
b. Zn + Pb(NO₃)₂ --> Zn(NO₃)₂ + Pb
c. Zn + Pb(NO₃)₂ --> Zn(NO₃)₂ + Pb

5. 
a. Double replacement
b. AgNO₃ + NaCl --> AgCl + NaNO₃
c. AgNO₃ + NaCl --> AgCl + NaNO₃

6. 
a. Single replacement
b. K + Ba(C₂H₃O₂)₂ --> KC₂H₃O₂ + Ba
c. 2 K + Ba(C₂H₃O₂)₂ --> 2 KC₂H₃O₂ + Ba

7.
a. Double replacement
b. LiOH + Fe(NO₃)₃ --> Fe(OH)₃ + LiNO₃
c. 3 LiOH + Fe(NO₃)₃ --> Fe(OH)₃ + 3 LiNO₃

8.
a. Single replacement
b. CuO + H₂ --> Cu + H₂O
c. CuO + H₂ --> Cu + H₂O

9.
a. Single Replacement
b. K + H₂O --> KOH + H₂
c. 2 K + 2 H₂O --> 2 KOH + H₂