Give reasons : Concentrated H2SO4 cannot be used instead of dilute H2SO4 in laboratory preparation of hydrogen gas why ?

Because concentrated sulfuric acid is a strong oxidizer, so it has to be in water to react with metals and produce hydrogen properly. Sometimes dilute sulfuric acid is used, but that is another story

1. and 2. question: in which equation? or i don't understand you, 3. a), 4. c), 5. c), 6. b), 7. d), 8. d) if small number is underlined(behind symbol) , b) if big number is underlined(in front of symbol)9. what is underlined there? 10. c)