(i) Carbon and hydrogen:
CH4 (methane)
C2H6 (ethane)
(ii) Nitrogen and magnesium:
Magnesium nitrate
Formula: Mg3(NO3)2
(iii) Sodium and phosphorus:
Sodium phosphate
Formula: Na3PO4
(iv) Potassium and oxygen:
Potassium oxide
Formula: K2O
(v) Boron and oxygen:
Boron oxide
Formula: B2O3