16.2. Solution Concentration http://www.ck12.org
- Weigh out the appropriate amount of solute, and place it in a volumetric flask.
- Fill the flask about halfway with solvent, and swirl until the solute is completely dissolved.
- Finally, add enough solvent so that the meniscus of the solution lines up with the calibration mark on the flask.
FIGURE 16.10
Example 16.9
You want to make a 0.154 M solution of sodium chloride (the approximate concentration of a standard saline
solution). Describe how you would prepare 1.00 L of this solution.
Answer:
Step 1: Calculate
The molarity of our solution is going to be 0.154 M, and the total volume will be 1.00 L. Plugging these values into
the definition of molarity will allow us to calculate the necessary moles of solute.
M=
mol
L
mol=M×L
= ( 0 .154 M)×( 1 .00 L)
= 0 .154 mol NaClBecause we will be measuring out NaCl by mass, we need to determine the mass of 0.154 moles of NaCl using its
molar mass (58.44 g/mol).
0 .154 mol NaCl×^581 .mol NaCl^44 g NaCl= 9 .00 g NaCl
Step 2: Weigh out 9.00 g of NaCl and place it in a 1 L volumetric flask.
Step 3: Fill the flask about halfway with water, and swirl until the NaCl is completely dissolved.
Step 4: Add more water until the meniscus of the solution lines up with the calibration mark. Mix well.
Molality
Another way to express the concentration of a solution is by determining its molality. Themolality(m) of a solution
tells us how many moles of solute are combined with each kilogram of solvent. Note that there are two differences