13-21 p. 181

13.  a.6 moles
      b. 5 moles
      c. 5 moles

14. a. 1 mole
      b. 621grams
      c. 28 grams
      d. 223 grams

15. The percent oxygen by mass is not 67% because oxygen weighs more.

16. a. 216 + 32= 248   216/248 x 100= 87%
      b. 54 + 48 = 102    54/102 x 100= 53%
      c. 40+12+48= 100  40/100x100= 40%

17. a. 207+32+ 64= 303   207/303x100= 68%
      b. 10%
      c. 6%
   
18. The difference between reusing and recycling is that recycling is converting waste into usable material, and reusing is to use more than once.

19. a. renewable resources: solar energy, wind power, tides, hydroelectricity

      b. nonrenewable resources: petroleum, coal, natural gas, nuclear energy

20. a. reusing
      b. recycling
      c. reusing

recycling plant

21. They are both made, packed, shipped, used, disposed, and hopefully recycled.