termdescriptionexample solute: substance which dissolves solid sodium chloride, NaCl(s) solvent: substance which enables the solute to dissolve liquid water, H2O(l) solution: prepared by dissolving a solute in a solvent NaCl(aq) is NaCl(s) dissolved in H2O(l)
| amount of solutevolume of solutionconcentration | ||
| moles mol | litres L | moles per litre (molarity) mol L-1 (mol/L or M) |
concentration of a stock solution in mol L-1 = moles of solute ÷ volume of solution in litres
c1 = n1 ÷ V1
c1 = molarity of stock solution (concentration of stock solution in mol L-1)
n1 = moles of solute dissolved (in mol)
V1 = volume of stock solution (in L)
The dilution equation (dilution formula or dilution expression) is:
c1V1 = c2V2c1 = concentration of stock solution (before dilution) in mol L-1
V1 = volume of stock solution present before dilution in L
c2 = concentration of new dilute solution (after more solvent added) in mol L-1
V2 = total volume of the new dilute solution in L
Note: V2 > V1 and c2 < c1
findcalculation determine concentration of dilute solution c2 = c1V1 ÷ V2 determine volume of dilute solution V2 = c1V1 ÷ c2 determine concentration of stock solution before dilution c1 = c2V2 ÷ V1 determine volume of stock solution used (before dilution) V1 = c2V2 ÷ c1
The dilution equation (dilution formula or dilution expression) is:
c1V1 = c2V2c1 = concentration of stock solution (before dilution) in mol L-1
V1 = volume of pipette used to transfer the stock solution in L
c2 = concentration of new dilute solution (after water added) in mol L-1
V2 = volume of volumetric flask used to prepare the dilute solution in L
findcalculation determine concentration of dilute solution c2 = c1V1 ÷ V2 determine volume of dilute solution V2 = c1V1 ÷ c2 determine concentration of stock solution c1 = c2V2 ÷ V1 determine volume of pipette used V1 = c2V2 ÷ c1
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Dilution Technique
Dilution means to reduce the concentration of a solution.A solution can be diluted by adding solvent to a given volume of stock solution.
In order to dilute a solution we need:
- A container of solution of known concentration.
This is the stock solution, the solution to be diluted. - A pipette to remove a known volume of this solution from the container and transfer it to a new container such as a volumetric flask.
- A clean volumetric flask.
- A container of pure solvent, the same solvent used to make the original solution, such as water.
- A way of adding the pure solvent to the volumetric flask, for example a glass funnel and a pasteur pipette.
| 1 | The solution to be diluted will be in a vessel such as a volumetric flask. This solution is referred to as the stock solution. In order to prevent contamination of this stock solution, place a small amount of it in a clean, dry, conical flask (erlenmeyer flask), swirl it around and then discard this solution appropriately. DO NOT use water or any other solvent to rinse the flask because adding more solvent would dilute the solution and we would no longer know what the concentration of this solution is. Pour some of your stock solution into the rinsed conical flask. Stopper the conical flask when you are not using it to prevent contamination from the atmosphere. Label it with the date, and the formula of the solution and its concentration, for example [NaCl(aq)] = 0.10 mol L-1 Record the concentration of this undiluted stock solution as c1 in mol L-1 | c1 = [undiluted solution] mol L-1 |
| 2 | You will need a clean, dry, pipette with a known volume. This will be printed on the pipette, for example 10.00 mL, 25.00 mL, etc Use the pipette filler (bulb) to draw up a small amount of stock solution from the conical flask, remove the pipette from the solution, swirl the solution in the pipette around to rinse it and then discard this solution suitably. DO NOT use water or any other solvent to rinse the pipette because you would be diluting the solution and we would no longer know what the concentration of this solution is. Place the rinsed pipette into the stock solution in the conical flask and draw up the solution until it is past the level marked on the pipette. Remove the pipette from the stock solution. Suspended the pipette within the confines of a beaker. Allow the solution to escape from the pipette drop by drop until the bottom of the meniscus (when viewed at eye level) sits just on the level marked on the pipette. You need to record the volume of this undiluted, stock solution, held in the pipette. It is a good idea to convert this volume in milliltres (mL) to a volume in litres (L) if your concentration is mol L-1 (molarity): volume in L = volume in mL ÷ 1000Record the volume of this pipette as V1, for example, V(NaCl)1 = 10.00 mL = 10.00/1000 = 0.0100 L V1 = [volume of undiluted solution in pipette] L | V1 = V(undiluted solution) L |
| 3 | Position the pipette so that the tip of the pipette meets the inside neck of the clean volumetric flask at an angle. Allow the solution in the pipette to escape and run down the neck of the volumetric flask into the body of the flask. Pipettes are usually designed to deliver a known volume, this means that the small drop of solution remaining in the tip of the pipette is supposed to be there, so don't try to blow it out. We now have a known volume of the undiluted stock solution, with a known concentration in this volumetric flask. Calculate the moles of solute, n(solute), contained in this volumetric flask. moles of solute (mol) = concentration of solution (mol L-1) × volume of solution delivered from pipette (L) n(solute) = c1 × V1 Record the moles of solute, n(solute) in mol. | n(solute) = c1 × V1 mol |
| 4 | Place a glass funnel in the neck of the volumetric flask. As you pour your solvent, for example water, into the funnel, leave a small air gap between the stem of the funnel and the neck of the volumetric flask so that the solvent flows freely. When the solvent reaches the base of the neck of the volumetric flask, stop pouring and allow any solvent remaining in the funnel to flow through. Slowly, and carefully, add more solvent until the bottom of the meniscus is about 1 cm from the level marked on the neck of the volumetric flask when viewed at eye level. Use a clean pasteur pipette that has been rinsed with the solvent to add more solvent drop-by-drop to the volumetric flask until the bottom of the meniscus sits exactly on the level marked on the neck of the flask when viewed at eye level. Stopper the volumetric flask. Record the volume of the diluted solution in this volumetric flask as V2 in mL Convert the volume in millilitres to a volume in litres dividing by 1000. Record this volume as V2 L. | V2 = [dilute solution] L |
| 5 | Calculate the concentration of the dilute solution, c2: concentration of dilute solution = moles of solute ÷ volume of dilute solution molarity of dilute solution (mol L-1) = moles solute (mol) ÷ volume of dilute solution (L) c2 = n(solute) ÷ V2 Affix a label to the volumetric flask which states the date the solution was made, its formula and concentration. | c2 = [dilute solution] mol L-1 c2 = n(solute) ÷ V2 mol L-1 |
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| undiluted solution | → pipette | → volumetric flask | → add solvent | → dilute solution |
| c(undiluted solution) mol L-1 | V(pipette) in mL V(pipette) in mL ÷ 1000 = V(pipette) L | n(solute) = c(undiluted solution) × V(pipette in L) mol | V(dilute solution) in mL V(dilute solution in mL) ÷ 1000 = V(dilute solution in L) | c(dilue solution) = n(solute) ÷ V(dilute solution in L) |
| c1 in mol L-1 | V1 in L | n(solute) = c1 × V1 in mol | V2 in L | c2 = n(solute) ÷ V2 in mol L-1 or c2 = (c1 × V1) ÷ V2 in mol L-1 |
So we now have an equation (expression or formula) to calculate the concentration of a solution after it has been diluted:
| c1 = concentration of undiluted solution in mol L-1 V1 = volume of stock solution added to the volumetric flask = volume of pipette used in L c2 = concentration of the dilute solution in mol L-1 V2 = volume of volumetric flask holding the dilute solution in L |
If we rearrange this equation (expression or formula) by multiplying both sides of the equation by V2 we get:
c1V1 = c2V2
which simply means that the moles of solute transferred by pipette and placed in the volumetric flask (n1) equals the moles of solute present in the volumetric flask after more solvent was added during the dilution (n2).
| moles solute transferred by pipette to volumetric flask | = | moles solute in volumetric flask after solvent added (dilute solution) |
| c1V1 | = | c2V2 |
| n1 | = | n2 |
This equation (formula or expression) is very useful because we can rearrange it find:
- c1 the concentration of the undiluted stock solution:
c1 = c2V2 ÷ V1
- V1 the volume of pipette required to transfer the undiluted solution to the volumetric flask for dilution:
V1 = c2V2 ÷ c1
- V2 the volume of volumetric flask required to make the dilute solution:
V2 = c1V1 ÷ c2
- c2 the concentration of solution in the volumetric flask after dilution:
c2 = c1V1 ÷ V2
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Calculate the new concentration in mol L-1 (molarity) if enough water is added to 100.00 mL of 0.25 mol L-1 sodium chloride solution to make up 1.5 L.
- What is the question asking you to do?
Calculate concentration of NaCl(aq) in mol L-1 after dilution
c2 = ? mol L-1 - What information (data) has been given in the question?
Extract the data from the question :
c1 = 0.25 mol L-1
V1 = 100.00 mL = 100.00 mL ÷ 1000 mL/L = 0.10 L (volume must be in litres)
V2 = 1.5 L - What is the relationship between what you know and what you need to find out?
Write the dilution equation (dilution formula or dilution expression):
c2 = (c1V1) ÷ V2 - Solve the problem:
Substitute the values into the dilution equation and solve :
c2 = (0.25 × 0.10) ÷ 1.5 = 0.017 mol L-1
[NaCl(aq)] = 0.017 mol L-1 (or 0.017 mol/L or 0.017 M)
Calculate the volume of solution after dilution (V2 = (c1V1) ÷ c2)
Calculate the volume in litres to which 500.00 mL of 0.020 mol L-1 copper sulfate solution must be diluted to make a new solution with a concentration of 0.0010 mol L-1.
- What is the question asking you to do?
Calculate the volume of CuSO4(aq) after dilution in L
V2 = ? L - What information (data) has been given in the question?
Extract the data from the question :
c1 = 0.020 mol L-1
V1 = 500.00 mL = 500.00 mL ÷ 1000 mL/L = 500.00 × 10-3 L = 0.50 L (since there are 1000 mL in 1L)
c2 = 0.0010 mol L-1 - What is the relationship between what you know and what you need to find out?
Write the dilution equation (dilution formula or dilution expression):
V2 = (c1V1) ÷ c2 - Solve the problem:
Substitute the values into the dilution equation and solve :
V2 = V(CuSO4)new = (0.020 × 0.50) ÷ 0.0010 = 10 L
Calculate the concentration in mol L-1 of solution before dilution c1 = (c2V2) ÷ V1
Calculate the concentration of the undiluted CuSO4(aq) if 10.00 mL of this solution was used to make 100.00 mL of dilute solution with a concentration of 0.20 mol L-1.- What is the question asking you to do?
Calculate the concentration of the undiluted CuSO4(aq) in mol L-1
c1 = ? mol L-1 - What information (data) has been given in the question?
Extract the data from the question :
V1 = 10.00 mL = 10.00 mL ÷ 1000 mL/L = 10 × 10-3 L = 0.010 L (since there are 1000 mL in 1 L)
c2 = 0.20 mol L-1
V2 = 100.00 mL = 100.00 mL ÷ 1000 mL/L= 100 × 10-3 = 0.10 L - What is the relationship between what you know and what you need to find out?
Write the dilution equation (dilution formula or dilution expression):
c1 = (c2V2) ÷ V1 - Solve the problem:
Substitute the values into the dilution equation and solve :
c1 = c(CuSO4)undiluted = (0.20 × 0.10) ÷ 0.010 = 2.00 mol L-1
Calculate the volume of the pipette used to dilute a solution (V2 = (c1V1) ÷ c2)
Calculate the volume of the pipette in millilitres used to transfer a quantity of 0.15 mol L-1 CuSO4(aq) to a 250.00 mL volumetric flask made up with enough water to produce a solution with a concentration of 0.012 mol L-1.- What is the question asking you to do?
Calculate the volume of the pipette in millilitres
V1 = ? mL - What information have you been given in the question?
Extract the data from the question :
c1 = 0.15 mol L-1
c2 = 0.012 mol L-1
V2 = 250.00 mL = 250.00 mL ÷ 1000 mL/L= 0.25 L - What is the relationship between what you know and what you need to find out?
Write the dilution equation (dilution formula or dilution expression):
V1 = (c2V2) ÷ c1 - Solve the problem:
Substitute the values into the dilution equation and solve :
V1 = V(CuSO4)undiluted = (0.012 × 0.25) ÷ 0.15 = 0.020 L
0.020 L = 0.020 L × 1000 mL/L = 20 mL
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Chris the chemist requires two aqueous solutions of sodium carbonate, Na2CO3(aq), of different concentrations for an analytical procedure. Chris carefully weighs out 5.00 g of sodium carbonate and transfers this to a 500.00 mL volumetric flask, adding enough water to make the solution up to the mark. Then Chris uses a pipette to transfer 50.00 mL of this solution to a 250.00 mL volumetric flask, adding enough water to make the solution up to the mark.Calculate the concentration of the second solution in mol L-1.
Solving the Problem
Using the StoPGoPS model for problem solving:
| STOP! | State the question. | What is the question asking you to do? Calculate the concentration of the second solution in mol L-1. What chemical principle will you need to apply? Apply stoichoimetry (chemical calculations) What information (data) have you been given?
|
| PAUSE! | Plan. | Step 1: Calculate moles of Na2CO3 used to make stock (undiluted) solution Assume the Na2CO3 is 100% pure (no impurities) Step 2: Calculate concentration of the stock (undiluted) solution n(Na2CO3) from above Step 3: Calculate the moles of Na2CO3 in the pipette which is transferred to the second volumetric flask for dilution cstock calculated above Step 4: Calculate concentration of dilute solution of Na2CO3 in second volumetric flask. n(Na2CO3 pipette) = n(Na2CO3 second volumetric flask) calculated above |
| GO! | Go with the Plan. | Step 1: Calculate moles of Na2CO3 used to make stock (undiluted) solution Assume the Na2CO3 is 100% pure (no impurities) Step 2: Calculate concentration of the stock (undiluted) solution n(Na2CO3) from above = 0.04717 mol Step 3: Calculate the moles of Na2CO3 in the pipette which is transferred to the second volumetric flask for dilution cstock calculated above = 0.0943 mol L-1 Step 4: Calculate concentration of dilute solution of Na2CO3 in second volumetric flask. n(Na2CO3 pipette) = n(Na2CO3 second volumetric flask) calculated above = 0.00472 mol |
| PAUSE! | Ponder Plausability. | Have you answered the question that was asked? You were asked to find the concentration of the second (dilute) solution, you have done this, so yes, you have answered the question that was asked. Is your solution to the question reasonable? A dilute solution will have a lower concentration than the undiluted stock solution. |
| STOP! | State the solution. | The concentration of the second solution is 0.019 mol L-1. |
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