Density of a sol (handout)

To learn how to make volume and mass measurements, create solutions, and determine the density of solutions graphically and mathematically.

Scientific information is often presented in quantitative forms as measurements.
Skills needed include:
- Designing experiments for accurate measurements.
- Correct use of lab equipment to minimize errors.
- Understanding limitations of scientific measurements.

Numerical Value:
- The digit(s) that represent the measurement.
Unit of Measurement:
- Indicates the scale (e.g., grams for mass, mL for volume).
Estimate of Uncertainty:
- Reflects the precision of the measurement; recorded as significant figures.
- The last digit is an estimated or "doubtful" digit.
- Guideline: Measure to one place beyond the smallest marked scale division on measuring devices.

Meniscus:
- Liquid surfaces curve downwards in cylindrical glassware due to surface tension, and readings should be taken from the bottom of this curve.
- It is crucial to align your eye with the meniscus to get accurate readings.
Graduated Cylinder:
- Typically used to measure liquid volumes accurately.
- Available in varying capacities (e.g., 10 mL, 100 mL).
- Scale divisions depend on the size:
  - A 100-mL graduated cylinder is divided into 1 mL increments.
  - A 10-mL graduated cylinder is divided into 0.1 mL increments.
- The scale is read to one digit beyond the smallest division.

Pipets:
- Glass tubes used to transfer measured amounts of liquid.
- Can be calibrated (multiple volumes) or volumetric (single volume).
- Usage Instructions:
1. Rinsing a Pipet:
  - Use a beaker with the liquid you will pipet (or DI H2O for cleaning).
  - Draw up liquid in the pipet, swirl to wash the walls, and drain.
2. Obtaining Measured Liquid:
  - Ensure the beaker contains more liquid than needed to avoid air bubbles.
  - Insert the pipet tip just above the bottom of the beaker.
  - Employ a pipet bulb to draw the solution into the pipet, keeping it above the intended volume mark.
  - Use your finger to control the flow and achieve the desired volume.
  - Transfer liquid by placing the tip against the wall of the receiving container at an angle.
3. Finish by Tapping:
  - Tap the pipet tip to remove any excess liquid, avoiding forced expulsion from the pipet.

Solution Definition:
- Homogeneous mixtures where a solute is dissolved into a solvent (usually water for aqueous solutions).
- Steps to Make a Solution:
1. Determine the volume and concentration needed.
2. Weigh the solute and measure the solvent.
3. Combine solute and solvent ensuring complete dissolution.
Using a Volumetric Flask for Accurate Solutions:
1. Weigh the solute and place it in the flask.
2. Fill roughly half full with DI H2O.
3. Shake to dissolve and fill to the meniscus line with water.
4. Stopper and shake again for thorough mixing.
Concentration by Percent Mass Formula:
$% ext{solute} = rac{ ext{mass of solute (g)}}{ ext{mass of solution (g)}} imes 100$

Definition of Dilutions:
- Achieved by adding solvent, decreasing the solute concentration.
Dilution Process Steps:
1. Calculate the stock solution volume needed (using the dilution equation).
  $C<em>1V</em>1 = C<em>2V</em>2$
  Where:
- C = concentration (usually in molarity, M)
- V = volume.
1. Fill volumetric flask half full with solvent.
2. Add the calculated stock solution.
3. Accurately fill to the meniscus line and shake to mix.

Definition of Density:
- A property comparing mass to volume.
- Density formula:
  $ext{Density} = rac{ ext{mass (g)}}{ ext{volume (mL)}}$
Characteristics of Density:
- A physical property that does not change based on sample size.
- Remains constant for a substance at a specific temperature.
- Used for identifying substances.

Assigned sugar or salt solutions with concentrations of:
- 0.0%, 5.0%, 10.0%, 15.0%, and 20.0%.
Use DI H2O as the 0.0% solution.
Calculate amounts needed to make:
- 100.0 mL of 5.0% and 15.0% solutions from solid.
- 100.0 mL of 10.0% and 20.0% solutions by dilution.
Prepare all solutions using a volumetric flask.

Measure a 10.0 mL aliquot of each known solution and DI H2O into a beaker, weighing beforehand.
Weigh the 10.0 mL aliquots.
Repeat the above steps in triplicate for accuracy.

Measure a 10.0 mL aliquot of the unknown solution into a clean beaker and weigh beforehand.
Weigh the aliquots in triplicate for reliability.

Part B – Known % Mass Determinations
- Columns include:
  - Mass of Empty Beaker
  - Mass of 10.0 mL solution + beaker (Trial 1, 2, and 3)
  - Density (g/mL) for each concentration.

Determine density of known sugar/salt solutions using average mass for calculations.
Graph known sugar/salt solutions:
- Y-axis = density
- X-axis = % sugar/salt.
Determine density of unknown solutions.
Use graph for determining % sugar/salt of the unknown solution(s).
Analyze the graph trend (direct/inverse proportionality).

Why was it instructed to prepare extra known solutions?
Convert grams of sugar/salt weighed out to moles, and calculate the concentration of the solution in molarity (moles/liter).
Evaluate the reliability of a student’s analysis stating an unknown salt solution is 27% salt, based on lab parameters.