We wanted to know how many kilograms of hydrogen Joanne the scientist has. We know that she has 9kg of pure H₂O, no isotopes. The atomic weight of oxygen is 16, and the atomic weight of hydrogen is 1. Since there are 2 hydrogen atoms in H₂O, the ratio of hydrogen to oxygen weight is 2:16, or 1:8. This makes the hydrogen portion of total weight 1/9. Since Joanne has 9kg of water, she has exactly 1kg of hydrogen!