Sodium Chloride Solutions
This experiment has two parts.
PART 1 - You prepare a solution of sodium chloride (NaCl) in water to a specified molarity.
PART 2 - You obtain a different sodium chloride solution, whose molarity is also known, and you calculate
the mass of NaCl contained in a 25.0 mL sample of the new solution. You then measure out this volume,
and remove the water from the solution to see if it actually contains the amount of NaCl you calculated.
To begin, get an assignment card. The card states your assignments for the molarity of the solution you
will prepare in Part 1.
Part 1 – Preparing a Sodium Chloride Solution of Accurately Known Molarity
1. First obtain a 250 mL volumetric flask from the fumehood. Before using the flask, rinse out any
chemicals that might be on the walls of the flask by adding some deionized water, swirling, and pouring out
the deionized water. (It’s OK for the flask to be wet for further use in the experiment, as long as it’s just
wet with pure water.)
2. Next, calculate the number of grams of sodium chloride (NaCl) that should be added to the flask and
dissolved in water to make a solution of the molarity that is stated on your assignment card. Then weigh
out the calculated amount of sodium chloride into a beaker (250-mL beaker or larger). The easiest way to
do this is to place the beaker on the balance pan and press the “zero” button (“On Tare”). This sets the
mass of the beaker to zero and you can directly read the mass of the NaCl that you add to it. Add NaCl
slowly using a scoopula until the mass in the beaker is very close to the calculated mass.
Sample calculation for calculating mass of NaCl needed to make 250 mL of 3.00M NaCl solution:
? g NaCl = 250 mL solution x 3.00 mol NaCl x 58.44 g NaCl = 250 x 3.00 x 58.44 = 43.8 g NaCl
1000 mL solution
1 mol NaCl
1000 x 1
3. Add deionized water to the beaker to dissolve the sodium chloride. The volume of water that you add
should be somewhat less than the volume of the volumetric flask. Stir the salt-water mixture until the NaCl
dissolves (but if it doesn’t all dissolve, you can still proceed to the next step.)
4. You now want to get ALL of the sodium chloride into the volumetric flask. First, pour the salt solution
from the beaker into the volumetric flask. You may want to use a funnel for this. (If some of the NaCl
hasn’t dissolved, try to pour just the liquid into the flask, leaving the solid NaCl in the beaker.) Then rinse
the beaker with deionized water (from a plastic wash bottle) and empty the rinse water into the volumetric
flask. (If some solid NaCl is still in the beaker, the rinse water should dissolve this NaCl.) Repeat the rinsing
until you are satisfied that the beaker is wet only with rinse water and not with NaCl solution but be sure to
limit the amount of rinse water you use so that you don’t fill the volumetric flask above the line.
5. Now add deionized water to the volumetric flask to fill it EXACTLY up to the line on the neck. The flask is
properly filled to the line when the bottom of the meniscus (curve) of the water in the neck of the flask is
on the line when viewed exactly horizontally from the level of the line.
6. Put the cap on the volumetric flask and thoroughly mix the solution by inverting the flask several times.
You now have a sodium chloride solution whose molarity is very accurately known.
7. On a piece of tape, write out a label stating the molarity of the solution and attach the label to the flask.
If this was a solution that your lab would use over a long period of time, you would want a better storage
container and a more permanent label … but this solution will only be used once.
Part 2 – Determining the Mass of NaCl in a 25.0 mL Sample of a Solution with Known Molarity.
In Part 2, you obtain a different NaCl solution and you calculate the mass of NaCl solution contained in a
25.0 mL sample of the solution. You then measure out this volume of solution into an Erlenmeyer flask and
leave it in the oven so that the water will evaporate, leaving only solid NaCl.
1. Obtain (from another student) a solution of sodium chloride in water whose molarity is accurately
known. Calculate the mass of NaCl contained in a 25.0 mL sample of this solution.
Sample calculation for mass of NaCl in 25.0 mL of a 4.20M solution:
? mass NaCl =
2. Obtain a clean dry empty Erlenmeyer flask. (The flask can be either a 250-mL flask or a 125-mL flask.) Put
a label on the flask so you will be able to identify it in the oven. Weigh the labeled flask on the top-loading
balance. Then, using a 25 mL volumetric pipette, carefully transfer 25.0 mL of the solution into the flask.
3. Put the flask in the lab oven. The oven should be no warmer than 80⁰C. Leave the flask in the oven until
the next lab period.
4. When the water has evaporated from the flask and only solid sodium chloride remains in the flask, you
can remove the flask from the oven, allow it to cool to room temperature, and weigh it again. Calculate the
mass of the salt in the flask, and calculate the percent recovery. It should be very close to the mass on
your assignment card.
Calculating percent recovery:
At the end of the experiment, all sodium chloride solutions can be disposed of by pouring down any drain.
The solid sodium chloride in the flask can be disposed of by adding water to the flask to dissolve it and then
washing it down the drain. You should thoroughly rinse out the volumetric flask before returning it to the
stockroom.