We know that the molarity of a solution is calculated by the following equation:
[tex]Molarity = \frac{# moles}{L} [/tex]
That being said we are given two of the 3 things that we need: Volume of water, as well as the molarity of the solution. Let's plug those in to the equation to find how many moles of [tex] Na_{2}S [/tex] that we need:
[tex]0.2 M = \frac{moles}{0.2 L} [/tex] =>[tex]moles =0.04[/tex]
This means that we have 0.04 moles of [tex] Na_{2}S [/tex].
In order to calculate the number of grams that we need for this solution, we must first calculate how many grams of [tex] Na_{2}S [/tex] are in 1 mole. We do this by taking the atomic masses of each element from the periodic table and adding them together.
Na = 22.99 g
S = 32.06 g
We have 2 Na's, and we have 1 S. So lets add them together:
2(22.99) + (32.06) = [tex] \frac{78.04g}{mol} [/tex].
Since we need 0.04 moles of [tex] Na_{2}S [/tex], we can multiply the molar weight of the molecule times the amount of moles needed to find the total grams that we need for the solution:
[tex]\frac{78.04g}{mol}*0.04 moles = 3.1216 g[/tex]
Now we know that in order to make a 0.2 M solution of [tex] Na_{2}S [/tex], we must use 3.1216 grams.
