A medical solution used in a hospital has `0.9%` NaCl by mass. How many grams of NaCl are present in `100 g` of solution?

By the definition, one percent of some quantity is a one hundredth of it. Hence `0.9%` of some quantity `Q` is  `0.9/100 Q = 9/1000 Q.`

In the given problem `Q = 100 g,` therefore `0.9%` of it is equal to

`9/1000*100 g = 9/10 g = 0.9 g.` This is the answer.

By the way, this concentration of  `NaCl`  is used in hospitals because it is close to human blood and tears by the osmotic pressure.