Common constituent in all acid solutions

I just need answer A.S.A.P

16.  The common constituent in all acid solutions is:  
  O2  
H+  
OH-  
SO4  

17. The chemical formula MgSO4.7H2O indicates  
  A titratable base  
A tritratable acid  
A hydrated compound  
A concentrated solution  

18. An ideal gas is at standard temperature and pressure. Which statement is false concerning this situation?  
  The temperature is at 20 degrees Celcius.  
The pressure is at one atmosphere.  
The barometer reading is at 760.  
One mol of the gas occupies 22.4 liters.  

19. Identify the major ionic species present in an aqueous solution of CaCO3.  
  Ca2+, C2-, O3  
Ca+, C4+, O32-  
Ca2+, C+, O2-  
Ca2+, CO32-  

20. A chemistry student prepares a solution as follows:
1. Water is added to the appropriate container up to the volume mark.
2. The chemical is accurately weighed and transferred to the container using a funnel.
3. The solution is thoroughly mixed.
What step is out of sequence for the correct preparation of a solution?  
  Step 1  
Step 2  
Step 3  
The steps are in the correct sequence.