1) trihydrate = . 3 H2O
2) octahydrate = . 8 H2O
Now, I know you may have just said three and eight to yourself. That's OK. Just remember the business with the dot AND putting H2O.
One really silly mistake I've seen is where the kid multiplies everything out. For example, with trihydrate, the person writes H6O3. So, what they did was take the three which should be out front and distributed it over the H2O.
Nah! No! Nein! Non!
You get the point.
Right?
3) Name this substance: MgSO4 . 9 H2O
4) Write the formula for: barium chloride dihydrate
Answers:
3) Magnesium sulfate nonahydrate
4) BaCl2 . 2 H2O