# Calculating Initial Concentrations

1. Jul 22, 2009

### PrincessJee

1. The problem statement, all variables and given/known data

I am trying to calculate the initial concentrations in a rate law lab

If I have the following
0.020M Br2 solution, and am using 5.0mL of it
4.0M acetone, and am using 5.0ml of it
1.0M HCl and am using 5.0ml of it
H2O 10.0ml

how do I calculate the initial concentrations of solutions used in each run (this example is the first run only).
After mixing all the reagents the total volume is 25.0mL, and the concentrations need to be calculate according to this volume?

2. Relevant equations

n=m/M
M=mol/L

3. The attempt at a solution
this is what I am guessing... but not sure if it is correct...
If I have 0.020M Br2
then that means I have 0.020mol of Br2
which means if I use 5 ml of it, my concentration will be (0.020mol/0.005L) = 4.0M

is this correct? I am very confused!!!

2. Jul 22, 2009

### Ygggdrasil

The equation you have to remember is:
MiVi = MfVf

For example, if you use have 5.0mL (Vi) of 0.020M Br2 (Mi) and dilute it to a final volume of 25mL (Vf), then you would setup the equations as follows:

(5.0mL)(0.020M) = (25mL)Mf

You can then solve for the final concentration (Mf) and see that it is 0.0040M.

The approach you tried is wrong because 0.020M means that you have 0.020 mol of Br2 in 1L. In 5mL, you would have only (0.020 mol/L) * (0.0050 L) = 0.00010 mol. This amount in 25mL gives a concentration of (0.00010 mol)/(0.025 L) = 0.0040 mol/L = 0.0040 M.

You could have immediately figured out that your initial guess (4.0M) was wrong because diluting a solution will never make the concentration increase!