Analía Bellizzi – Chemistry Classes

Ronald Reagan Senior High School

## 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

Discussion:

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

1. Using a digital balance measure exactly 17.55 g of NaCl from the vial that you received into a 100 cm3 beaker.
2. 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.
3. Transfer the solid to a 100 mL volumetric flask.
4. Wash the beaker, glass rod and funnel with some distilled water to be sure all the solid was transferred to the flask.
5. 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.
6. Fill with distilled water until the mark.
7. Mix again.

Before going on,

1. Calculate what is the percentage of salt in the solution prepared. This number is called %m/m
2. Calculate the concentration of the solution in moles/dm3
3. Transfer the solution into a beaker labelled STOCK SOLUTION
4. Rinse the volumetric flask with tap water and then with distilled water twice.

Part 2: DILUTION:

1. Extract the 25 mL from the STOCK SOLUTION
2. transfer that into a different volumetric flask.
3. add distilled water up to the neck of the flask and mix it thoroughly.
4. add distilled water again until the mark very carefully.
6. 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.

1. Calculate what is the percentage of salt in the diluted solution. This number is called %m/m
2. 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.

Density tower:

• Density tower:

1. After calculating the density of each, predict which liquid will float over each other.
2. Use one drop of food coloring to dye your solutions.
3. 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.
4. Repeat with the diluted solution.
5. Then, transfer 10 mL of distilled water.
6. Make a diagram of the solutions layers
7. CALL ME TO SEE YOUR TOWER.

Rinse everything with water and then with distilled water.

Place them in the rack to dry.

#### Your lab report should have the following parts: – READ COMPLETELY –

• Date
• Title
• purpose(s)
• materials
• procedures (It can be just showing the steps in the lab setup)
• lab setup (one for the solution preparation and other for the dilution process
• calculations Just below each section
• density tower diagram labelling the concentrated and diluted solution.

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#### VERY IMPORTANT

• Failing to complete ALL exercises or questions will result in an “I” (incomplete) which is equivalent to an “F”
• Everything  MUST be written in pen (Blue or black only)
• NO TYPED WORK WILL BE ACCEPTED
• Graphs & Charts MUST BE AS FOLLOWS:
• In graph paper.
• with LABELLED AXES AND UNITS using ruler.
• They must be pasted on the homework page or lab report using only tape or glue (NO STAPLED GRAPHS WILL BE ACCEPTED)
Any LAB SETUP DIAGRAM showing a process must be done in COPY PAPER and PASTED into the lab report