Molarity of Solutions: Making Dilutions
Name_________________________ Hr___
Solutions of known molarity are often available in the laboratory. You can use these to make new solutions of
new concentrations by just adding more solvent. As you observed in the previous laboratory activity, the
number of moles of solute does not change when a solution is diluted.
𝐦𝐨𝐥𝐞𝐬 𝐬𝐨𝐥𝐮𝐭𝐞
Since MOLARITY(M) =
, then by rearranging the equation, moles solute = M x L of solution
𝐋 𝐨𝐟 𝐬𝐨𝐥𝐮𝐭𝐢𝐨𝐧
moles of solute before dilution = moles of solute after dilution, or:
M1V1 = M2V2
Example: A student needs to prepare 100.0ml of 0.500M NaOH. He has a stock bottle of 3.00M NaOH. How
does he prepare the desired solution?
First determine the volume of 3.00M NaOH needed:
3.00M x V1 = 0.500M x 100.0ml
V1 =
(0.500M)(100.0mL)
3.00M
= 16.7ml
Measure 16.7ml of 3.00M NaOH into a
graduated cylinder. Mix with water, and
continue to add water until the final
volume is 100.0ml.
Concept Practice:
A. Explain why the resulting solution is always of lower concentration than the original.
B. Your experiment requires 5.0ml of 1.0M KOH. You have 1.0L of 0.50M KOH on the shelf. Can you
prepare the required solution? Explain your answer.
Problems:
1. How do you prepare 250.0ml of 0.20M NaCl from a stock of 1.0M NaCl?
2.
How do you prepare 400.0ml of 1.00M Zn(NO3)2 from a stock of 2.5M Zn(NO3)2?
3. What is the resulting concentration when 400.0ml of 12.0M H2SO4 is diluted to a new volume of 500.0ml?
4. What is the resulting concentration when 20.0ml of 6M HCl is added to 100.0ml of water? (This is a little
tricky. Think about the total volume of the new solution.)
5. What volume of 2.00M LiCl is needed to make 500.0ml of 0.35M LiCl?
6. What is the molarity of a solution made by adding 50.0ml water to 150.0ml of 0.10M KOH? (This is also
tricky.)
7. What volume of 12.0M HCl is needed to prepare 450.0ml of 2.00M HCl? How much water would be
needed?
8. Determine the concentration of a solution made by diluting 75.0ml of 5.0M NaOH with 200.0ml of water.
(tricky)
9. Determine the concentration of stock solution used when 40.0ml of it are needed to make 800.0ml of 0.20M
HC2H3O2.
10. Determine the volume of 1.00M KBr needed to prepare 500.0ml of 0.20M KBr.