Question

How much Sr(OH)2 • 8 H2O (M = 265.76) is needed to prepare 250.0 ML of solution in which [OH]= 0.100M?

Answer 1

Use the formula 
first step: 
Use the formula 
molarity= mole/liter
change ml to l
plug in data
 to get .1=mole/.25 or .1M*.25liter
which =.025 moles
then divide .025 moles by two because there are two OH in Sr(OH)2 
then multiply that by 265.76 (the molar mass of water) 
.0125*265.76
which is 3.32grams this is your answer 

Answer 2

The answer is = 3.322grams

The explanation:

1-First we need o get the no.of moles:

moles = molarity * volume(L)

by substitution:

∴ moles = 0.1M * 0.25 L

= 0.025 moles

but we have 2 OH in Sr(OH)2

so moles = 0.025 /2 = 0.0125 moles

2- then we need to get how many grams of it so, we are going to use this formula:

mass = moles * molar mass

by substitution:

∴ mass = 0.0125 * 265.76

= 3.322 grams