The directions for preparing a solution of 6.0M barium hydroxide depend on the volume desired. Since the volume isn't specified I'll explain how to prepare 500. ml (0.500L) of solution. You can easily substitute another volume into the calculation.
Let's start with the mathematical definition of molarity: It's moles of solute per liter of solution. You need to calculate the number moles of barium hydroxide needed, then convert that to grams so that it can be measured out.
1. Since molarity = moles solute/liters solution,
moles solute = (molarity)(liters of solution) = (6.0M)(0.500L) = 3.0 moles
2. Next, you need to know the molar mass of BaO to convert to grams:
137.3 g/mol + 16.0 g/mol = 153.3 g/mol
(3.0 moles BaO)(153.3 g/mol) = 460 grams BaO needed
3. Now that you know the required number of grams, weigh out this amount and use a funnel to put it into a volumetric flask of the volume you wish to prepare, in this case 500 ml.
4. Add deionized or distilled water to the flask to a point just short of the 500 ml line.
5. Stopper the flask, invert and swirl to mix.
6. Add additional water a drop at a time to bring the volume up to the 500 ml line and mix again.
Note that you don't measure out a specific volume of water, you add it until the desired total volume is reached. The presence of a solute sometimes adds to the volume so that you would need less than 500 ml of water, and sometimes causes the volume to contract so that you need more than 500 ml of water.
You can use a different volume in liters in step one. This will give you a different value for moles and mass of BaO.
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