SOLUTIONS – DILUTIONS
Upon completion of this lab, the student should be able to
- proficiently calculate molarities for solutions.
- prepare a solution of known concentration.
- prepare a dilute solution from a more concentrated one.
- perform serial dilutions.
- use volumetric and Mohr pipets and a volumetric flask
Solutions are an important part of chemistry. In this lab you will practice preparing solutions of different concentrations. The amount of solute that is dissolved in a given quantity of solvent is expressed as the concentration of the solution. A dilute solution contains only a small amount of solute in a given amount of solution. The unit chemists use most often to describe concentration of solutions are molarity units. The molarity, M, of a solution is the number of moles of solute in one liter of solution. To determine the molarity of a solution, the following equation can be used:
Part 1: PREPARING A STOCK SOLUTION
- Using a digital balance measure exactly 17.55 g of NaCl from the vial that you received into a 100 cm3 beaker.
- Add distilled water to dissolve the NaCl in around 50-70 cm3 (DO NOT OVERPASS 80 mL) of distilled water and stir using the glass rod until all the solid has dissolved.
- Transfer the solid to a 100 mL volumetric flask.
- Wash the beaker, glass rod and funnel with some distilled water to be sure all the solid was transferred to the flask.
- Top and mix thoroughly until no solid is seen. Important: Use your thumb or forefinger to secure the cap onto the flask while inverting. Do not shake the flask hard as the glass neck may break.
- Fill with distilled water until the mark.
- Mix again.
Before going on,
- Calculate what is the percentage of salt in the solution prepared. This number is called %m/m
- Calculate the concentration of the solution in moles/dm3
- Transfer the solution into a beaker labelled STOCK SOLUTION
- Rinse the volumetric flask with tap water and then with distilled water twice.
Part 2: DILUTION:
- Extract the 25 mL from the STOCK SOLUTION
- transfer that into a different volumetric flask.
- add distilled water up to the neck of the flask and mix it thoroughly.
- add distilled water again until the mark very carefully.
- cap the flask
- Mix it so the solute is evenly distributed, and the concentration of the solution is the same within the flask.
Important: Use your thumb or forefinger to secure the cap onto the flask while inverting. Do not shake the flask hard as the glass neck may break.
- Calculate what is the percentage of salt in the diluted solution. This number is called %m/m
- Calculate the Calculate the concentration of the diluted solution in moles/dm3
Part 3: Density of the solutions
- Zero the balance you will use.
- Measure the mass of an empty 10 cm3 graduated cylinder. RECORD THIS MEASUREMENT
- Using a plastic pipette, transfer between 8 and 9 mL of the stock solution into the graduated cylinder.
- Using a balance, measure the mass of the cylinder and the liquid. RECORD THIS MEASUREMENT.
- Calculate the mass of the liquid in the graduated cylinder
- By using the scale in the cylinder, record the volume with the correct amount of sigfig.
- Calculate the density of the liquid using the formula Density = mass / volume
- Repeat the process for the diluted solution.
- After calculating the density of each, predict which liquid will float over each other.
- Use one drop of food coloring to dye your solutions.
- By using the 10 mL pipette, transfer 10 ml of the concentrated solution VERY SLOWLY and using the wheel into a 50 mL graduated cylinder.
- Repeat with the diluted solution.
- Then, transfer 10 mL of distilled water.
- Make a diagram of the solutions layers
- CALL ME TO SEE YOUR TOWER.
Rinse everything with water and then with distilled water.
Place them in the rack to dry.