Answer: Water
Explanation: During photosynthesis, plants take in carbon dioxide (CO2) and water (H2O) from the air and soil
The arrangement in space and the interatomic distances and angles of the atoms in crystals, usually determined by x-ray diffraction measurements
Protons and neutrons.
Note: They probably are ABOUT the same mass. Don't think they are exactly the same.
Hope this helps!
Answer: 17) d. 
18. c. The empirical formula of a compound can be twice the molecular formula.
Explanation:
Molecular formula is the chemical formula which depicts the actual number of atoms of each element present in the compound.
Empirical formula is the simplest chemical formula which depicts the whole number of atoms of each element present in the compound.
To calculate the molecular formula, we need to find the valency which is multiplied by each element to get the molecular formula.
The equation used to calculate the valency is:

The empirical mass can be calculated from empirical formula and molar mass must be known.
17. Thus the empirical formula of
should be 
18. The molecular formula will either be same as empirical formula or is a whole number multiple of empirical formula. Thus the empirical formula of a compound can never be twice the molecular formula.
Tips
- write cue cards on respiratory system
- get someone to test you on your notes (friend,Mum,sister,brother ect)
- copy down diagrams of the system
- get someone to test you on Digram
- play online interactive games to test your knowledge on the respiratory system
- go through your textbook write a checklist things you know things you need to improve on.
- watch YouTube clips on the respiratory system