Based on the balanced equation
CH3CCH + 4O2 → 3CO2 + 2H2O

calculate the number of excess reagent units remaining when 38 CH3CCH molecules and 164 O2 molecules react?


Based on the balanced equation
Mg + 2HCl → MgCl2 + H2

calculate the number of excess reagent units remaining when 13 Mg atoms and 22 HCl molecules react?

Based on the balanced equation
2Mg + O2 → 2MgO

calculate the number of MgO formula units formed when 2 Mg atoms and 3 O2 molecules react? 

From the balanced equation
1 molecule of CH3CCH reacts 4 molecules of O2
Hence
38 molecules will react with 38 CH3CCH X 4 molecules of O2 =152 molecules of O2
No of molecules of O2 available in reacting mixture are 164
so O2 is in excess and CH3CCH is the limiting reagent
No of molecules in excess = 164-152=12
2)From balanced equation
1 atom of Mg reacts with 2 molecules of HCl
13 atoms of Mg reacts with 2X13 =26 molecules of HCl
so Mg is in excess because we have only 22 molecules of HCl
22 molecules of HCl can react with  22 X 1/2 atoms of Mg = 11 atoms of Mg
Excess reagent left after the reaction = 13-11=2 atoms of Mg

3) from the balanced equation :
2 atoms of Mg reacts with 1molecule of O2 to give 2 formula units of MgO
O2 is in excess .Limiting reactant is Mg .
2atoms of Mg gives two formula units of MgO .