Question

How many grams of Na2s do we need to dissolve in 200ml of water to get a solution that is 0.2 M?

Answer 1

We know that the molarity of a solution is calculated by the following equation:

$Molarity = \frac{# moles}{L}$

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 $Na_{2}S$ that we need:

$0.2 M = \frac{moles}{0.2 L}$ =>$moles =0.04$

This means that we have 0.04 moles of $Na_{2}S$.

In order to calculate the number of grams that we need for this solution, we must first calculate how many grams of $Na_{2}S$ 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) = $\frac{78.04g}{mol}$.

Since we need 0.04 moles of $Na_{2}S$, 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:

$\frac{78.04g}{mol}*0.04 moles = 3.1216 g$

Now we know that in order to make a 0.2 M solution of $Na_{2}S$, we must use 3.1216 grams.