Hi, Isabella,
The definition of Molar is moles/liter. We have a 0.150M solution of cesium hydroxide. If we had a liter, we'd have 0.150 moles of CsOH. But we don't. A thrifty 400 ml will work, but it takes a little more calculating. To find out how many moles are in 400ml, multiply the 400ml times the concentration, but we need to use the same units. M is moles/liter, so lets change 400ml to liters. 400ml/(1000ml/liter) = 0.400 liters.
Now we can multiply: (0.150 moles/liter)*(0.400 liter) = 0.060 moles CsOH. To find grams, we need the molar mass of CsOH, which I calculate to be 150 g/mole. Multiply the molar mass times the number of moles to get mass (in grams):
0.060 mole* 150 g/mole = 9 grams
I hope this helps,
Bob
