SUBDOMAIN 204.3- CHEMISTRY
Objective 204.3.4-6
“How To Make a Solution of Known Concentration”
By Nienke Adamse
A stock HCl solution is a more concentrated solution that needs to be diluted to
acquire the needed 5% HCl solution. Going from a 37% solution to a 5% solution we
need to dilute the stock solution with water.
37% solution means that 37g HCl (dry mass) has been dissolved in 100g of water. A
5% solution means that we need 5g of HCl dissolved in 100 g of water. This would
be the same as 37g HCl dissolved in 37g/5g x100g =740g of water.
A typical dilution calculation involves determining how much water must be added
to an amount of stock solution to get a solution with the desired concentration.
Since only water is added and not solute, we know that the moles of solute after the
dilution=moles of solute before the solution. The central idea in calculating the
amount of stock solution that is needed is the following equation:
M1V1  M2V2

M represents the molarity, and V the volume of the solution.
Because we work with percent solutions and because we need to know the mass of
the solution, we can adapt this equation to a more general one:
C1m1  C2m2

C represents the percent concentration, and m the mass of the solution. 1= before
the dilution, 2 is after the dilution.
Filling out the amounts into the equation:
(Mass percent= mass of solute/mass of solution x 100%)
37% x a (g)= 5% x 500 (g) a= the unknown mass of stock solution needed.
a=0.05x 500g/ 0.37
a= 67.6 g of stock 37%HCl solution
The amount of water that needs to be added to make 500 g is:
500-67.6= 432.4 g of pure water.
1 g of pure water= 1 ml of pure water
To make 500g of a 5% aqueous HCl solution from a 37% HCl stock solution, you can
use the following procedure:
Needed:
Measuring pipette, 5.0 ml
Graduated cylinder, 500 ml
37% HCl solution in Erlenmeyer flask
Empty Erlenmeyer flask
Water in wash bottle
Beaker, 500 ml
Glass stirring rod
1kg precision scale (resolution 1 mg)
Safety goggles
Safety apron
Workbench
Procedure:
Wear goggles and safety apron to protect yourself, HCl is a strong acid!
Step 1: Add the water from the wash bottle to the graduated cylinder until it
indicates 430 ml.
Step 2: Pour the water from the cylinder into the beaker.
Step 3: Fill the measuring pipette with 2.4 ml of pure water.
Step 4: Slowly add 2.4 ml water from the measuring pipette into the beaker.
Step 5: Put an empty Erlenmeyer flask on a 100 g precision (digital) scale and set
the scale on zero.
Step 6: Carefully pour the 37% HCl solution into the Erlenmeyer flask until the scale
indicates 65 g. Use the measuring pipette for the last 2.6 g. The scale should now
indicate 67.6 g
Step 7: Carefully pour the 67.6 g of stock solution into the beaker with 432.4 g of
pure water.
Step 8: Stir the diluted solution well with the glass stirring rod.
Remember: Always add acid to water when you dilute an acid!
REFERENCES
Zumdahl, S.S and Zumdahl, S.A (2009) Chemistry. Seventh
Edition
Boston, MA: Houghton Mifflin Company