In the chemical equation
2 HgO → 2 Hg + O₂
what is the mole ratio of O₂ to HgO?
The line in the image is the line that you would draw if you wrote the mole ratio as a fraction

2 HgO → 2 Hg + O2 If 0.57400 moles of HgO are reacted, how many moles of O₂ will be produced?
In the chemical equation
2 Fe(OH)₃ → Fe₂O₃ + 3 H₂O
what is the mole ratio of Fe(OH)₃ to H₂O?

If 2.5855 moles of H₂O are produced, how many moles of Fe(OH)₃ were reacted?
2 Fe(OH)₃ → Fe₂O₃ + 3 H₂O