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Practical laboratory calculations and procedures for creating solutions of specific concentrations.
Imagine you are a forensic scientist; if your reference solution is off by even 0.01%, an innocent person could be wrongly convicted. How do chemists ensure their 'recipes' are perfectly precise every single time?
To create a solution from a solid, we must determine the moles () required based on the target Molarity () and Volume ( in Liters). The fundamental relationship is . Once we have the moles, we convert to mass () using the Molar Mass () of the substance: . In the lab, precision is paramount; even a few extra drops of water can change the concentration and ruin an experiment. We always use an analytical balance to measure the mass to at least two or three decimal places to ensure the resulting chemical reactions are predictable and safe.
Calculate the mass of () needed to prepare of a solution.
1. Convert volume to Liters: . 2. Calculate moles needed: . 3. Calculate mass: .
Quick Check
If you need to make 1.0 L of a 2.0 M solution, how many moles of solute are required?
Answer
2.0 moles.
A standard solution is prepared using a volumetric flask, which is calibrated to contain one specific volume. 1. Weigh the solute precisely on an analytical balance. 2. Dissolve the solid in a beaker with a small amount of distilled water (less than the final volume). 3. Transfer the liquid into the volumetric flask using a funnel. 4. Rinse the beaker and funnel with distilled water to ensure every particle enters the flask. 5. Dilute by adding water until the bottom of the meniscus touches the etched line. 6. Invert the stoppered flask several times to ensure the solution is homogeneous. Never fill the flask directly to the line with solid inside, as the solid occupies volume!
Quick Check
Why do we rinse the beaker and funnel into the volumetric flask after the initial transfer?
Answer
To ensure that all of the measured solute is transferred into the flask, maintaining the calculated concentration.
How would you prepare of from a stock solution?
1. Identify variables: , , . 2. Rearrange for : . 3. Calculate: or . 4. Procedure: Measure of stock and add water until the total volume is .
If you add of water to of a solution, what is the new molarity?
1. , . 2. Calculate : . 3. Solve for : .
Which piece of glassware is specifically designed to prepare a solution of a fixed, precise volume?
You need to dilute a stock to make of solution. What is ?
In the equation , the variable represents the volume of water added to the stock solution.
Review Tomorrow
In 24 hours, try to explain to a peer why the number of moles doesn't change during a dilution even though the concentration does.
Practice Activity
Find a household cleaner label (like bleach) and try to calculate how much water you would need to add to a 100mL sample to cut its concentration in half.