Вычислите массовые доли элементов в следующих соединениях: а) FeS2, б) Ag2O, в) СuСl2 г) Ca3N2.

12. He didn't hear the thunder during the storm last night because he was sleeping.
13. We hope that scientists will find solutions to our most urgent problems.
14. By next decade people will have created more efficient purification equipment.
15. What do geologists study?
16. There isn’t a cloud in the sky. It is going to be a lovely day.
17. This time last year I was swimming in the Black Sea.
18. They are working at a chemical laboratory now.
19. In 1974, scientists didn't predict an earthquake in China.
20. They will be carrying out an experiment from ten till twelve o’clock in the morning.
21. He had solved this problem by 10 o’clock yesterday.