To calculate the number of moles of NaCl required to make 250 mL of a 3.00 M NaCl solution, we need to use the formula for molarity:
Molarity Formula
Molarity (M) is defined as the number of moles of solute per liter of solution. The formula is:
Molarity (M) = Moles of solute (mol) / Volume of solution (L)
Using this formula, we can calculate the moles of NaCl needed if we know the desired molarity and the final volume of solution.
Calculating Moles of NaCl
In this problem, we know:
- Desired molarity (M) = 3.00 M
- Volume of solution (L) = 0.250 L
To calculate moles of NaCl:
Molarity (M) = Moles of NaCl (mol) / Volume of solution (L)
3.00 M = Moles of NaCl (mol) / 0.250 L
Solving this gives:
Moles of NaCl = (3.00 M) x (0.250 L)
Moles of NaCl = 0.750 mol
Conclusion
Therefore, the number of moles of NaCl required to make 250 mL of a 3.00 M NaCl solution is 0.750 mol.
Summary
To summarize:
- Molarity depends on the moles of solute and the volume of solution.
- Use the molarity formula to calculate moles from molarity and volume.
- For a 3.00 M NaCl solution with a volume of 0.250 L, 0.750 mol of NaCl is required.
Understanding how to use molarity and moles allows calculating the quantities of compounds needed for chemical solutions. Molarity and moles are fundamental concepts in chemistry that are applied across many laboratory and industrial applications.
Molarity Definitions and Formulas
Let’s take a deeper look at some key definitions and formulas related to molarity:
Molarity
Molarity (M) is a concentration unit that measures the number of moles of solute dissolved per liter of solution:
Molarity (M) = moles solute (mol) / liters solution (L)
Moles
A mole is a standard unit equivalent to 6.022 x 1023 particles of a substance (Avogadro’s number).
Dilution Formula
The dilution formula is used to calculate new solution volumes or concentrations:
M1V1 = M2V2
- M1 = initial molarity
- V1 = initial volume
- M2 = final molarity
- V2 = final volume
Using Molarity in Calculations
Some examples of using molarity in chemical calculations:
- Calculating reactant/product amounts for reactions
- Determining solution concentrations needed for experiments
- Diluting stock solutions to working concentrations
- Determining solution molality or mass percent
- Calculating solution properties like freezing point depression or osmotic pressure
Understanding molarity is essential for labs and industrial applications in chemistry, biology, pharmaceuticals, and more fields.
Practice Examples
Let’s work through some examples of using molarity:
Example 1
How many moles of potassium chloride are needed to make 2.50 L of a 2.00 M KCl solution?
Solution:
- Molarity (M) = 2.00 M
- Volume (V) = 2.50 L
- Use formula: Molarity (M) = moles solute (mol) / Volume solution (L)
- Rearrange to solve for moles KCl:
- Moles KCl = (Molarity) x (Volume)
- Moles KCl = (2.00 M) x (2.50 L) = 5.00 mol
Example 2
What volume of 4.50 M HCl solution can be made with 325 mmol of HCl?
Solution:
- Given: 325 mmol HCl
- Convert mmol HCl to moles HCl: 325 mmol x (1 mol / 1000 mmol) = 0.325 mol HCl
- Molarity (M) = 4.50 M
- Use formula: Molarity (M) = moles solute (mol) / Volume solution (L)
- Rearrange to solve for Volume:
- Volume (L) = moles solute (mol) / Molarity (M)
- Volume (L) = 0.325 mol HCl / 4.50 M = 0.0722 L = 72.2 mL
Practice problems like these help reinforce how to use molarity equations for chemical calculations.
Dilution Calculations
Another important application of molarity is diluting solutions to specific lower concentrations. This requires using the dilution equation:
M1V1 = M2V2
Where:
- M1 = initial molarity
- V1 = initial volume
- M2 = final molarity
- V2 = final volume
Let’s try an example dilution calculation:
Example 3
If you need 150 mL of a 2.50 M NaCl solution, how much of a 5.00 M NaCl stock solution would be required?
Solution:
- M1 = 5.00 M (stock concentration)
- V1 = ? (volume of stock solution required)
- M2 = 2.50 M (final/diluted concentration)
- V2 = 0.150 L (final volume required)
- Use dilution formula:
- M1V1 = M2V2
- (5.00 M)(V1) = (2.50 M)(0.150 L)
- V1 = (2.50 M)(0.150 L) / (5.00 M) = 0.075 L = 75 mL
Therefore, 75 mL of the 5.00 M stock would be needed to create the 150 mL of 2.50 M solution.
Molarity, Molality, and Mass Percent
In chemistry, there are different ways to express solution concentrations:
- Molarity – moles solute per liter solution
- Molality – moles solute per kilogram solvent
- Mass Percent – mass solute per 100 g solution
While molarity is most common, it’s helpful to understand how to interconvert between these units:
Molarity to Molality
Molality (m) = moles solute (mol) / kilograms solvent (kg)
To convert molarity (M) to molality (m):
- Need to know solution density (d) in g/mL
- Molality (m) = (Molarity (M) x density (d)) / molar mass solvent (g/mol)
Molarity to Mass Percent
Mass percent = mass solute (g) x 100 / total mass solution (g)
To convert molarity (M) to mass percent:
- Need molar mass of solute (g/mol)
- Mass percent = (molarity (M) x molar mass (g/mol) x 100) / (1 + molar mass (g/mol))
Being able to interconvert between concentration units is a useful skill for chemistry calculations and analyses.
Uses and Applications of Molarity
Understanding molarity helps unlock many important chemistry applications and calculations. Some examples include:
Academic Labs
- Preparing solutions to execute experiments
- Diluting and serially diluting reagents
- Calibrating analytical instruments and techniques
- Analyzing reaction stoichiometry and theoretical yield
Industrial Labs
- Monitoring and controlling product quality
- Calculating production batch sizes and material requirements
- Standardizing analytical testing procedures
- Ensuring proper concentrations for safety and efficacy
Healthcare and Pharmaceuticals
- Preparing intravenous (IV) and infusion therapy solutions
- Formulating drug products at therapeutic concentrations
- Determining drug dosages and dilutions
- Quality control testing of raw materials and final products
Overall, molarity calculations facilitate quantitative chemistry critical for R&D, manufacturing, medicine, and more.
Molarity, Moles, and Chemical Calculations – Review of Key Points
Let’s summarize some key points about using molarity and moles for chemical calculations:
- Molarity (M) measures solution concentration as moles solute per liter of solution.
- Moles represent the number of particles based on Avogadro’s number.
- Use the molarity formula to interconvert between moles, molarity, and volume.
- Apply molarity to calculate reactant amounts, product yields, dilutions, and more.
- Practice calculation examples to improve molarity skills.
- Understand related concentration units like molality and mass percent.
- Molarity is widely used in chemistry research, industry, medicine, and everyday consumer products.
Mastery of molarity and mole calculations is an essential quantitative skill in chemistry across many fields and applications.
Frequently Asked Questions
What are some common mistakes when using molarity?
Some common molarity mistakes include:
- Forgetting to convert volume to liters
- Mixing up molar mass and molarity
- Improperly cancelling units in calculations
- Using the wrong volume in the dilution equation
- Calculating molarity incorrectly after mixing solutions
Always check your units and think about whether your answer makes logical sense.
What is the molarity of pure water?
Pure water has a molarity of 1 x 10-7 mol/L since water molecules dissociate into hydroxide and hydronium ions at this low concentration. For most purposes, pure water is treated as having zero molarity.
How do you make a 1 M solution from a solid solute?
To make 1 L of 1 M solution from a solid solute:
- Calculate the molar mass of the solute
- Determine the number of moles needed for 1 L of 1 M solution (this will be 1 mol)
- Weigh out the corresponding mass of solid using the molar mass
- Dissolve this mass in sufficient solvent to reach 1 L total volume
This ensures the final concentration will be 1 M as desired.
Conclusion
In summary, molarity and mole calculations are fundamental quantitative skills in chemistry. Learning the molarity formula, how to use moles, and applications like dilutions will provide you with a strong basis for performing chemical calculations needed in all areas of chemistry. Molarity allows converting between easily measured volumes and masses to molecular-level amounts represented by moles.
