Lesson 1. Solutions and Molarity

State of Matter 

In chemical equation, subscripts are used to indicate the state of matter: 
(aq) aqueous
(l) liquid 
(s) solid
(g) gas 
Example: H₂(g) + O₂(g) --> H₂O(l) 
NaCl(s) --> Na⁺(aq) + Cl^-(aq)

A compound that is dissolved in water is no longer in the solid, liquid or gaseous state, The subscript (aq) is written after the chemical formula of a compound is dissolved in water. 

Dilution

Most solutions are stored in concentrated form, and diluted for use when required. When an aqueous solution is diluted, the number of moles of solute does not change. Water is added to make the concentration smaller. Since the initial mole (n_i) equals the final mole (n_f) and n=CV, we can rearrange our formula as:  

n_i = n_f 
C_iV_i = C_fV_f

Let's see how this equation helps us when solving dilution problems. 

Example 1
If concentrated HCl(aq) has a concentration of 12.5 mol/L. What volume of concentrated HCl(aq) will be required to prepare 300 ml of 0.200 mol/ L diluted HCl(aq)? 

 
Solution: 
Step 1: Convert 300 ml to L = 0.300 L
Step 2: Calculate the volume of concentrated HCl(aq)

C_iV_i = C_fV_f 
Vi = CfVf/ Ci  
Vi= (0.200 mol/L x 0.300 L) / (12.5 mol/L) 
=0.0048 L = 4.8 mL  

Preparation of a Dilution

Write out the materials and procedures for preparing the dilution in the following problem. 
What volume of a 0.500 mol/L stock solution( original) of NaNO₃ is needed to prepare 100 mL of a 0.050 mol/L solution?

Solution: 
Step 1: Find the initial volume 
CiVi =CfVf 
0.500 mol/Lx Vi = 0.100 L x 0.050 mol/L 
Vi = 0.010 L = 10.0 mL
Step 2: Materials
NaNO₃ solution, beaker, volumetric flask, pipet bulb, wash bottle, funnel, medicine dropper. 
Step 3: Procedure:

1. Add initial volume to a 100 mL volumetric flask by pipet (in this case Vi=10mL)
2. Add distilled water to the volumetric flask and bring the solution to the 100 mL line.
3. Insert stopper to the flask, and invert twice or more to mix the solution.