How do you figure out molarity problems?

For homework, we have a list of problems that have to do with molarity. I've looked through my book to try and figure it out but I can't.


If you could just show me step-by-step how to do the problem I have below, that would really help! Thanks! Here's the problem:


How many grams of LiF would you need to dissolve to make a 2.5M solution with a volume of 2L? I know the first step is to convert LiF to grams, which is 26, but I don't know what to do after that.How do you figure out molarity problems?
Right. 26 gm of LiF is equal to ONE mole of Li F.


The definition of 1 M is 1 mole per liter. So, 26 gm of LiF/L is 1 M.


You need 2.5 M and you need 2 liters. If you needed 1 liter that would be 26gm x 2.5 = 65 gms/liter. But for 2 liters, you will need 2 x that or


130 gms. So you would dissolve 130 gms of LiF in enough water to make 2 liters.How do you figure out molarity problems?
You calculated the molar weight, which is needed (in the second step).





Start with 2 liters and multiply by the molarity (moles per liter) to get moles.





Multiply moles by the molar weight (grams/mole, which you calculated) to get grams needed.
The way that your teacher taught you to do it in class. Maybe if you paid better attention you would know.