Making solutions of a specified molarity is a common task in chemistry labs and classrooms. Molarity (M) indicates the concentration of a solute (such as NaCl) dissolved in a solvent (such as water). The molarity is calculated by dividing the moles of solute by the liters of solution. For example, a 3M solution contains 3 moles of solute per liter of solution.
To make a 3M NaCl solution, we first need to calculate the number of moles of NaCl required for the desired volume of solution. Then, using the molar mass of NaCl, we can convert moles of NaCl to grams. Finally, we can weigh out the required mass of NaCl and dissolve it in water to achieve the desired concentration.
In this article, we will walk through the full calculation to determine how many grams of NaCl are needed to make 250 mL of a 3M NaCl solution. We will explain the concepts and equations required at each step. Let’s get started!
Calculating Moles of NaCl
The first step is to calculate the number of moles of NaCl required. Recall that molarity is defined as:
Molarity (M) = moles solute / liters solution
We can rearrange this equation to solve for the number of moles:
moles solute = M x liters solution
For our problem, we know:
– Molarity (M) = 3 M
– Volume of solution (liters) = 0.250 L
Plugging these values into the equation:
moles NaCl = (3 M) x (0.250 L) = 0.75 moles NaCl
So, for 250 mL (0.250 L) of a 3M NaCl solution, we need 0.75 moles of NaCl.
Using Molar Mass to Convert Moles to Grams
Now that we know how many moles of NaCl are required, we need to convert this to a mass in grams using the molar mass.
The molar mass of a compound is equal to its atomic mass constants multiplied by its molecular formula. For NaCl, the molar mass is:
Na: 22.99 g/mol
Cl: 35.45 g/mol
NaCl: 58.44 g/mol
Using the molar mass, we can convert moles of NaCl to grams:
grams NaCl = (moles NaCl) x (molar mass NaCl)
Plugging in our values:
grams NaCl = (0.75 moles NaCl) x (58.44 g/mol) = 43.8 g NaCl
Therefore, to make 250 mL of a 3M NaCl solution, we need 43.8 grams of NaCl.
Summary
To summarize:
1. Calculate moles of NaCl needed using the molarity and volume of solution.
moles NaCl = (Molarity) x (Liters of solution)
2. Convert moles of NaCl to grams using the molar mass.
grams NaCl = moles NaCl x molar mass NaCl
3. For 250 mL (0.250 L) of 3M NaCl:
– Moles NaCl = (3 M) x (0.250 L) = 0.75 moles
– Grams NaCl = (0.75 moles) x (58.44 g/mol) = 43.8 grams
Therefore, to make 250 mL of a 3M NaCl solution, 43.8 grams of NaCl are required.
Explanation of Calculations
Let’s take a closer look at the calculations step-by-step:
Moles of NaCl Calculation
Molarity (M) = moles solute / liters solution
Rearrange to solve for moles:
moles solute = M x liters solution
Plug in the values for our problem:
Molarity (M) = 3 M
Volume of solution (L) = 0.250 L
moles NaCl = (3 M) x (0.250 L) = 0.75 moles NaCl
This makes sense because the molarity tells us there are 3 moles of NaCl for every 1 liter of solution. Since we have 0.250 liters, which is 1/4 of a liter, we should calculate 1/4 of the moles in 1 liter:
(3 moles/1 L) x (0.250 L) = 0.75 moles NaCl
Converting Moles to Grams
Use the molar mass of NaCl to convert moles to grams:
NaCl molar mass = 58.44 g/mol
grams NaCl = (moles NaCl) x (molar mass NaCl)
Plugging in the values:
moles NaCl = 0.75 (from previous calculation)
grams NaCl = (0.75 moles NaCl) x (58.44 g/mol NaCl) = 43.8 g NaCl
So 0.75 moles of NaCl is equivalent to 43.8 grams of NaCl based on its molar mass.
How to Make the 3M NaCl Solution
Now that we calculated we need 43.8 grams of NaCl, how do we actually make the solution? Follow these steps:
1. Measure 43.8 grams of NaCl solid
Using an analytical balance, weigh out 43.8 grams of NaCl. Rock salt or table salt are fine to use.
2. Transfer NaCl to a beaker or flask
Pour the NaCl into a clean 250 mL beaker or Erlenmeyer flask. The flask should have volume markings.
3. Add a small amount of water and swirl to dissolve
Add about 50 mL of distilled water to the flask. Gently swirl the flask until the NaCl dissolves.
4. Add additional water up to the 250 mL line
Once the NaCl is fully dissolved, add additional distilled water until it reaches the 250 mL line on the flask.
5. Cover and mix the solution
Cover the flask with parafilm and invert several times to mix thoroughly. The NaCl should fully dissolve to make a clear, homogeneous solution.
6. Use the 3M NaCl solution
Your 3M NaCl solution is now ready to use for experiments or other applications requiring a 3M concentration. Be sure to label the flask with the concentration.
Common Uses for 3M NaCl Solution
Some common uses and applications for a 3M NaCl solution include:
– Chemistry experiments studying ionic compounds, solubility, conductivity, etc.
– Modeling seawater conditions (3% salinity is similar to ocean water)
– Calibrating conductivity meters or other salinity-dependent instruments
– Protein precipitation or aggregation studies
– Modulating enzyme activity
– Mixing with agar to make salt concentration-specific microbiological media
– Standardizing ion-selective electrodes for chloride detection
– Chloride analysis by titration with silver nitrate
– Draw out myoglobin from meat in order to study muscle structure
So in summary, a 3M NaCl solution has wide utility for research and testing applications across many fields including biology, biochemistry, food science, oceanography, and environmental science.
Impact of Inaccurate NaCl Concentration
It’s important to accurately make the 3M NaCl solution because experimental results can depend on the precise concentration. Some potential impacts if the concentration is off:
– Solutions used to calibrate instruments would provide inaccurate calibrations if not at the right molarity. This could lead to unreliable experimental measurements.
– Enzyme activities may be higher or lower than expected since many enzymes are sensitive to ionic strength.
– Microorganisms may grow differently than anticipated if their expected salt tolerance is not matched.
– Protein precipitation would be inconsistent if the NaCl molarity does not match protocols.
– Titration results will be imprecise if the concentration is incorrect.
To avoid these pitfalls, proper solution preparation technique and calculations are key. Follow the step-by-step method outlined above to accurately weigh reagents, dissolve the NaCl, and dilute to the desired volume and concentration.
Conclusion
In conclusion, to make 250 mL of a 3M NaCl solution, 43.8 grams of NaCl are required. This is calculated by:
1. Determining the moles of NaCl needed using the molarity and volume of solution:
moles NaCl = (3 M) x (0.250 L) = 0.75 moles NaCl
2. Converting moles of NaCl to grams using the molar mass (58.44 g/mol):
grams NaCl = (0.75 moles) x (58.44 g/mol) = 43.8 g NaCl
The process involves accurately weighing the NaCl solid, dissolving it in a volumetric flask, then diluting to the final volume of 250 mL. The resulting 3M NaCl solution can be used for a variety of chemistry experiments and procedures requiring a standardized high salt concentration.
