# How do you prepare 100 mL of 0.2 M from 1m KCl?

Preparing solutions of specific molar concentrations is a common task in chemistry labs and research settings. In this case, we need to prepare 100 mL of a 0.2 M potassium chloride (KCl) solution from a 1 M stock solution. There are a few quick steps we can follow to calculate the proper dilution and prepare our target 0.2 M KCl solution.

## What is a Molar Solution?

Before going through the dilution procedure, it’s helpful to understand what molarity means. Molarity (M) is a concentration unit that measures the number of moles of solute present in one liter of solution. For example, a 1 M solution contains 1 mole of solute per 1 liter of solution.

• The units of molarity are moles per liter (mol/L).
• Molarity indicates the concentration of a solution, specifically the concentration of solute dissolved in solvent.
• Molarity depends on the number of moles of solute, not the mass.
• A 1 M solution contains 1 mole of solute dissolved in a total solution volume of 1 L.

Knowing these basics about molarity will help us calculate the proper amounts of stock and solvent needed to prepare our desired 0.2 M KCl solution.

## Step-by-Step Dilution Procedure

Here is a step-by-step overview of how to prepare 100 mL of 0.2 M KCl from a stock 1 M KCl solution:

1. Calculate the number of moles of KCl needed for 100 mL of 0.2 M solution using the formula:

Molarity x Volume = Moles

So for a 0.2 M solution with a volume of 0.1 L (100 mL):

(0.2 mol/L) x (0.1 L) = 0.02 mol KCl
2. Calculate the volume of 1 M stock solution containing 0.02 moles of KCl using the formula:

Moles = Molarity x Volume

Rearranged: Volume = Moles / Molarity

Volume of 1 M stock = 0.02 mol KCl / 1 mol/L = 0.02 L = 20 mL
3. Add 20 mL of 1 M KCl stock solution to a 100 mL volumetric flask
4. Fill the remaining volume of the flask with solvent (water) up to the 100 mL line
5. Mix thoroughly by inverting or stirring

By taking 20 mL of 1 M KCl stock solution and diluting it to 100 mL total volume with water, we have prepared 100 mL of 0.2 M KCl solution. The full calculations are shown below.

## Dilution Calculations

### Number of Moles Needed

Molarity x Volume = Moles

(0.2 mol/L) x (0.1 L) = 0.02 mol KCl

### Volume of Stock Solution

Moles = Molarity x Volume

Rearranged: Volume = Moles / Molarity

Volume of 1 M stock = 0.02 mol KCl / 1 mol/L = 0.02 L = 20 mL

## Summary

To prepare 100 mL of 0.2 M KCl from a 1 M stock solution:

1. Calculate the number of moles needed (0.02 mol)
2. Calculate the volume of 1 M stock solution containing this amount of moles (20 mL)
3. Add 20 mL of 1 M stock to a 100 mL volumetric flask
4. Fill to the 100 mL line with solvent (water)
5. Mix thoroughly

Using this simple dilution procedure allows us to accurately prepare the desired concentration of KCl solution for use in the lab. Proper dilution techniques are an essential skill in chemistry. Mastering these calculations ensures you will obtain your specific target molarity each time.

## Understanding Molar Concentrations

Preparing solutions of specific molar concentrations relies on an understanding of the quantitative relationships between solute moles, solution molarity, and solution volume. Here we’ll take a closer look at the meaning of molarity and go through some example calculations:

### What is Molarity?

• A concentration unit expressing the number of moles of solute per liter of solution (mol/L)
• Indicates the concentration of solute present in a certain volume of solvent
• Common way to express concentrations for liquid solutions
• Abbrv. M; can also see notation like 0.2 M or 1 M

### Key Relationships

• Molarity (M) = moles solute (mol) / Volume solution (L)
• Moles solute = Molarity (mol/L) x Volume solution (L)
• Volume solution = Moles solute (mol) / Molarity (mol/L)

### Examples

• 1 M solution = 1 mole solute per 1 liter solution
• 0.5 M solution = 0.5 mole solute per 1 liter solution
• 0.25 M solution = 0.25 mole solute per 1 liter solution

You can use these relationships to calculate any variable if you know the other two – a handy skill for preparing solutions!

## Dilution Strategy

When preparing a target molar concentration from a stock solution, we use a dilution strategy:

1. Calculate moles of solute needed for final volume and concentration using formula:
2. Moles = Molarity x Volume

3. Calculate volume of stock solution containing this amount of moles using formula:
4. Volume = Moles / Molarity

5. Measure out the calculated volume of stock solution
6. Add solvent until final desired volume is reached
7. Mix thoroughly

Following this dilution approach allows us to accurately prepare the desired concentration and volume of solution.

## Practice Examples

Let’s go through some practice examples of dilution calculations:

### Example 1

Prepare 50 mL of 0.1 M NaOH from a 0.5 M NaOH stock solution.

1. Moles NaOH needed:
Molarity x Volume = Moles
(0.1 mol/L) x (0.05 L) = 0.005 mol NaOH
2. Volume of 0.5 M NaOH stock:
Moles = Molarity x Volume
Volume = Moles / Molarity
Volume = 0.005 mol NaOH / 0.5 mol/L = 0.01 L = 10 mL
3. Take 10 mL of 0.5 M NaOH stock and dilute to 50 mL total with water. Mix well.

### Example 2

Prepare 200 mL of 0.4 M HCl from a 6 M HCl stock solution.

1. Moles HCl needed:
(0.4 mol/L) x (0.2 L) = 0.08 mol HCl
2. Volume of 6 M HCl stock:
Volume = Moles / Molarity
Volume = 0.08 mol HCl / 6 mol/L = 0.013 L = 13.3 mL
3. Take 13.3 mL of 6 M HCl stock and dilute to 200 mL total with water. Mix well.

Practice these types of calculations to become skilled at diluting solutions to specific molar concentrations. Having a solid understanding of molarity relationships is the foundation for success.

## Tips for Proper Dilution Technique

Follow these tips for proper technique when diluting solutions:

• Use volumetric glassware for accuracy
• Measure solvent first, then add stock solution
• Add stock solution slowly to avoid overshooting the mark
• Use a funnel if pouring into a narrow opening
• Rinse stock solution container to transfer entire sample
• Fill flask to the calibration mark with solvent
• Mix diluted solution thoroughly before use

Proper dilution technique ensures an accurate final molar concentration and volume of solution. Take care when diluting to avoid errors.

## Common Equipment for Dilutions

Certain glassware and equipment is designed specifically for diluting solutions accurately:

• Volumetric flasks – Flasks with narrow necks marked at a precise volume. Allow mixing known volumes.
• Graduated cylinders – Tall, narrow cylinders marked with volume graduations. Useful for measuring solvent volumes.
• Pipettes – Glass or plastic tubes used to measure and transfer small solution volumes.
• Burettes – Long graduated glass tubes with stopcocks. Can precisely dispense stock solutions.

Using the proper glassware helps ensure an accurate dilution to the target concentration. Volumetric flasks are ideal for the final diluted solution volume.

## Safety When Diluting Solutions

Lab safety should always be considered when diluting solutions:

• Wear PPE – gloves, goggles, lab coat
• Work in a fume hood when diluting hazardous substances
• Never pipette by mouth – use pipette bulbs/pumps
• Label diluted solutions clearly with concentration and date
• Dispose of chemical waste properly after dilution
• Avoid spills by pouring slowly and using funnels
• Rinse equipment thoroughly after diluting corrosive substances

Diluting a solution doesn’t necessarily make it harmless. Take appropriate precautions based on the risks of the chemical substances involved. Follow all safety guidelines and ask for assistance if needed.

## When to Use Diluted Solutions

Diluting stock solutions is necessary for:

• Lower concentration solutions for reactions/tests
• Matched concentrations for titrations
• Reduced concentrations for safe handling
• Specific concentrations for calibration curves
• Analysis methods requiring accurate molarities

Anytime you need an exact molar concentration different than your stock solution, performing a dilution calculation and careful technique is crucial.

## Conclusion

Diluting solutions from concentrated stock solutions to prepare specific molar concentrations is a fundamental lab skill. By understanding molarity units and relationships, practicing calculation strategies, and using proper techniques, chemists can accurately make the desired target concentration. Careful dilution helps ensure experimental success when an exact molarity is needed.